Undoubtedly, two attributes of truthfulness and honesty (truth and trust) are the most manifest signs of personality. Rather, we can not call a human the one who lacks these two, and both of them originate from a common source, because truthfulness is nothing else than trust in speech, as trust is nothing else than truth in action, and as we will see their social results are similar. Haply for this respect, "truth in speech" and "trusteeship" are mentioned together in the sayings of Islamic great leaders, and even in ordinary speech, truthfulness and honesty are accompanied.

Value and importance of truthfulness
As it was referred, truthfulness and honesty are the manifest signs of man's personality as well as magnificence of his mind and purity of his soul. Truthful people are usually brave, frank, sincere, disinterested, and free from wrong prejudices and excessive love and hatred, because truthfulness is not possible without them.
In contrast, liars are usually coward, hypocrite, greedy, fanatic, stubborn, and or involved in wrong love and hatred. Truthful people are certainly principled. Less happens such people to be guileful, erratic, and hypocrite, because honesty is not in concord with any of these affairs. As it will be described in incentives of lying, this attribute originates from one of the above vices, and on this account, we can consider truthfulness as a key for detecting interior of individuals in different aspects, and so truth in speech is called as a means of evaluation of people in the narrations and traditions of Prophet's House A.S.:
Imam Sadegh A.S.:
Do not regard one's long bowing and prostration, because it may be their habit, and if they leave it, are disturbed. Rather, look at their truthfulness and honesty.(1) Holy Quran says: And thou shalt certainly know them in the twisting of their speech.(2)

Miraculous results of truthfulness
In social point of view, the greatest service that truth and trust can offer to men is attracting confidence and reliance. Of course, we know that all scientific, industrial and economic developments are the results of his life system that is social life. But collaboration and consultation, which are the bases of social life, are only possible when individuals trust and rely on each other. Otherwise, people will have the problems of social life, disturbances, struggles, disputes and sabotages, without enjoying its advantages. Obviously, the most important and effective means of attracting reliance and confidence is "truth and trust", and its most dangerous enemy is lie. If economical firms lie only in introduction of one of their goods, people will suspect in their best products.
If the rulers lie, people will disregard their promises, invitations, and warnings, and they will lack powerful public support. If lie becomes popular in scientific environments, scientists and scholars will not trust on the speeches, narrations, and judgements of each other, and thus everybody should personally suffer all required labors for a scientific research, and could not use the results of thousands years of others' life. And this type of lie is the ugliest and dirtiest one.

Lie, source of all sins
In several narrations, it is referred to this reality that trust and truth cause purity of action, and lie is the key of all types of sins. For better understanding of effect of truthfulness and lie on the man's deeds and actions, first we refer to a number of narrations from Prophet's House A.S. in this regard, and then we will analyze it.
Imam Ali A.S. says:
Truthfulness guides people to goodness, and goodness invites to paradise.(3) Imam Sadegh A.S. says:
When a God's servant says the truth, God says: he said the truth and performed a good deed, and when he lies, He says: he lied and performed an evil deed!(4) Imam Askari A.S. says:
All vices are put in a room and its key is lie.(5) It is easily inferred from this tradition that lie is the key of all vices and evils. Now we study the effects of truthfulness and lie in man's deeds: 1- Lie is the source of hypocrisy, because truthfulness refers to harmony of tongue and heart, and lie is in harmony of these two, and hereof difference between man's interior and outward is started, and the liar gradually proceeds to full hypocrisy. Holy Quran refers to this fact and says:
So, as a result, He puts hypocrisy into their hearts, until the day they meet Him, for that they failed God in that they promised Him and they were liars.(6)
2- Lie is the tool of most sins. Dishonest and guileful, robber and treacherous, short weighing and hoarder, tricky and perjurer individuals can not continue anymore and reach to a stalemate if they do not lie.
3- An envious person for envying, a proud person for ostentation and frippery, a subservient and flatterer person for achieving his goals, greedy and secularist individuals for attaining their objectives, usually resort to die. They debase the envied person in public view falsely, and glorify themselves falsely. They say thousands of flatteries falsely, and resort to lie for satiation of their greedy souls.
4- One who is bound to truthfulness has no alternative save restraining himself from most of the sins, because he thinks that if he is asked about that issue, he shall say the truth, and if he says the truth, it will be disgraceful. So, he prefers to forget those deeds on the whole. Thus, being bounded to this attribute insures man against numerous sins.
5- Most of lies cause other lies or sins, because it is widely seen that liars say other lies for justifying their untrue subjects, and or perform other evil deeds for preventing disclosure of their lies.
It is clarified from what was mentioned above (considering the explanations mentioned in each part), that if a man is verily bounded to truthfulness and honesty, he has to leave most of sins because each of them somehow originates from lie. Rather, lie is the key of achieving them and a truthful man should recede from sin willingly or not.

Lie is not compatible with belief
It is deduced from different traditions that lie is not in concord with belief, and lying is the sign of a sort of unbelief. These traditions have been inspired by Holy Quran where it says: They only forge falsehood, who believe not in the signs of God, and those -–they are liars.(7) "Forge falsehood" may refer to deliberate falsehood.
And now some of the narrations:
The Prophet S.A. was asked: Is it possible for a believer to be coward? He answered: yes. It was asked: Is it possible for him to be stingy? He answered: yes. It was asked: Is it possible for him to be liar? He answered: no.(8) Imam Ali A.S. said: A man does not taste the taste of belief, unless he refuses lie, either seriously or for humor.(9) Avoid lie, which is not in concord with belief.(10)
And it is concluded from these narrations that believers avoid lie, and belief and lie do never accompany each other. This point refers to the fact that the liars do not easily accept invitation of the prophets based on self-comparison. Since they themselves lie in little affairs of life, they can not believe that prophets to be really truthful in such big affairs. They may be among the believers, but if we explore in depth of their hearts, we will find that it is not free from doubt and suspicion. Of course, this is true for those whose life is overshadowed by lie, and as interpreted in the tradition are liars.
The contrary is also true. That is to say the truthful people are often credulous, because they themselves are truthful and honest, and accept whatever they hear based on their superior nature, and so called they are "ear" unless they are notified. Liars have a special pessimism and suspicion on everybody and everything. They assume everything false, untrue, or at least suspicious. How it is possible for such people to believe in all speeches of prophets without any doubt and suspicion. So we see in the history that hypocrite, deviated and liar individuals often attributed lying to divine prophets.
The issue of self-comparison is one of the mental conditions in man, which could serve as the key for solving a lot of problems. It is often observed that unusual movements of criminal, thief and treacherous individuals acquaint the others. Since they are aware of their own condition, they suppose the same about the others, and so try to conceal themselves, and such unusual movements attract the attention of others.

Lie makes man improvident
A liar supposes that if he does not perform his different duties, can justify his violation through false excuses. For such person, not being faithful to promise and covenant, not observing time, not performing assigned duties and such like is easy, because he glosses over the issue with a series of false pretexts and saves himself from any rebuke. But in contrary, one who is bound to observing the principle of "truthfulness and honesty", is obliged to observe such affairs and so can not be improvident and careless.

The liar does not trust even in himself!
The liars not only do not trust in the others and assume them as themselves, but also do not trust in themselves, because they are suspicious to the reality of everything, including the degree of their ability and power when facing with the problems and different events of life as well as their determination and will power in performing the works. We read in aphorisms of Imam Ali A.S.: You should avoid making friend with a liar because he is like a mirage, making you feel remote things near and near things remote.(11) Although the matter of altering the realities is mentioned for the associates of a liar, but it is also formed in the lair as a result of getting accustomed to the lie, and they become suspicious and uncertain to the realities and facts related to themselves and the others, and this is one of the most painful conditions occurring for man in his life.

Source of lie
As it was already referred, lie originates from one of the mental weak points: Sometimes, a man lies for the fear of poverty, people leaving him alone and loosing his position and rank. Sometimes, he lies for the earnest to wealth, dignity, and other sensual desires, and uses this illegitimate means for achieving his purpose. Sometimes, passions as well as excessive love and hatred cause a man to say untruth in favor of one who loves or against one who is hated by him. Once he lies in different scientific, historical and such like issues for showing himself more than what he really is and expressing science and knowledge.
But all of these vicious attributes, which are the sources of lie, actually originate from one's low personality and weakness of spirit and belief. Those who do not believe on their own personality, and have a feeble and humble spirit, resort to lie, falsification, fraud and treachery for gaining their purposes and escaping losses. In contrary, powerful and high personality individuals rely on their own personality and ability, and seek their victory thereon. Moreover, those having a perfect belief on divine everlasting power, and considering His dominant will as the source of all blessings, victories and successes, have identified His power beyond all powers and His support beyond all supports, do not have any reason for resorting to untruth and lying for gaining a benefit or escaping a loss. And sometimes this dangerous vice roots in man for inattention to the disadvantages of lie and importance of truthfulness, and unhealthy family, or environment of society and associates.
One of the other important factors in lying is feeling low personality and inferiority complex. Those who are affected to such complex, try to compensate it with all types of lie, exaggeration and self-glorification.

Way of Treating lie
Taking into account the factors effective on formation of this attribute and its rooting, treatment procedure will be relatively clear, and in general for treating this moral deviation, we shall take following actions:
1- First of all, the liars should be informed about the painful consequences as well as spiritual, material, individual and social ill effects of this unfavorable vice, and it should be evaluated through meditation and thinking in Quran verses and traditions of great leaders of religion and aforementioned analyses and speeches of the great men that assumptive benefits of lie never counterweigh with such a lot of disadvantages. Moreover, the liars should be warned that if in some cases, lie has a personal benefit, it is temporary and very transitory, because no capital for a man in the society, in any condition and any rank,–is higher than the capital of people's trust and confidence on him, and the greatest enemy of this capital is lie.
It is noteworthy that some suppose that it is possible for a man to say some lies, never being disclosed and causing people's distrust. But this is a big mistake because experience has shown that in most cases, the untruthfulness of a liar is made clear, and its point is that any event has different relations with time, place, individuals and other events. One who wants to create an event, not externally existing, with his tongue, creates only one discrete event from among all other relations, and if he is very sharp and smart, he may already think about some other lies around it, and link the main false event with those events. However, he can never predict all possible relations with all other events, and consider their connections. So, most often after several questions, he fails to justify his speeches.
For example, consider the interesting judgement of Imam Ali A.S. about the young man whose father had traveled with a lot of wealth together with some people, and when returning, his companions claimed for his death. The accused persons, who were in fact the murderers of that young man's father, were disgraced very soon after Imam A.S. asked them several questions about the details of disease, death, shrouding and burying, as they had colluded for all together saying that he has become sick and died, but they had not colluded for where, what time of day and who has made ablution, and has shrouded, and has performed funeral prayer, and actually they could not collude for all of these matters. On this account, even the lie of the sharpest liar may be disclosed through a simple inspection.
Specially, the false and artificial relations, which a man creates for his lies, are not kept in his mind properly, because they are untrue, and if the liar is asked in different intervals, he involves in self contradiction and confusion, and this contradiction is one of the other factors for disclosing their lie. If it is commonly said that "a liar does not have a good memory", it may refer to the fact that whatever his memory may be strong, it is not possible for a liar to memorize everything!
2- Personification - One of the most effective ways of treating lie, is developing individuals' personality, because as we know, one of the important mental causes of lie is feeling inferiority and low personality, and in fact lying is a kind of reaction for compensating it. If the liars feel that they possess powers and talents through developing of which they can increase their own value and personality, they do not find any need for resorting to lie for developing a false personality in them. Moreover, they should be realized that social value of a truthful man, who has succeeded to attract "public confidence" through this moral virtue, is beyond all values, and his great spiritual capital of "social reputation" is the highest capital, with which no material capital can countervail, and he can provide all material facilities for himself using this capital.
Such person, not only is respected by people, but also enjoys a rank among the martyrs and prophets because in Quran the rank of truthful people is mentioned besides the rank of prophets and martyrs, where it says:
Whosoever obeys God, and the Messenger, they are with those whom God has blessed, Prophets, the truthful, martyrs, the righteous; good companions they are.(12)
The famous scholar, "Ragheb", in his book, "Mofradat" has narrated several meanings for "truthful" all proving this fact:
a- One who always says the truth.
b- One who never lies.
c- One who is truthful in his speech and idea, and his action proves his truthfulness.
3- Endeavor for strengthening foundations of belief in God in the heart of liars and notifying this fact to them that God's power is beyond all powers, and He is able to solve all problems for which weak faith individuals resort to lie.
The reliance point of truthful people when facing different events is God, and liars are alone in such cases.
4- The motives of lie and roots of this moral deviation including avarice, fear, self-conceit, excessive love and hatred and such like should be eradicated so that this dangerous vice can not find a favorable culture for its development in man.
5- The educational and association environments of liars should be cleaned from other liars so that gradually, according to the principle of "imitation" and impressibility, their own existence is purified of this vice.
This is so critical that it is narrated from Imam Ali A.S. in Islamic educational instructions: Lying is not proper, either for joke or seriously, and also it is not proper that one of you give a promise to his child, and then do not fulfill it.(13) It goes without saying that if parents are accustomed to truthfulness (even in giving small promises to their children), their child will never be a liar.

Lie in exceptional cases
The great jurists and ethic scholars following the traditions narrated in this respect, have mentioned some cases as exceptional from lie judgment, and have summarized some of them in two subjects: "exigency" and "reconciliation among people". It is narrated in a tradition from the Prophet S.A.: Swear falsely and save your innocent brother from death!(14) Herein, two points are notable:
First, all cases of exception actually return to one subject that lie is only permitted for more important expedients overshadowing its disadvantages, and of course, this is not restricted to "truthfulness" and "falsehood". Rather other forbidden things such as "eating carrion", "eating unclean food", "orphan molestation" and such like are permitted for survival or correction, and the cases mentioned in the traditions are some examples. For instance, since reconciliation among people and joining scattered hearts and washing out the dust of hatred and enmity from hearts is more important than the lie, which does not harm anyone else, it is permitted. The same is true in exigency cases for preserving life and honor of oneself or others, and or in planning and implementation of war plans, a lie, causing expedition in termination of war and preventing blood shedding and or repressing an unjust and corrupter enemy in this way, and also when two spouses argue about some matters, the continuation of which may result in their divorce or other corruption, but it is possible to solve their problem through one lie. In all of these cases, lie would be permitted for preservation of more important expedient affairs. If we pay attention accurately, in these cases, disadvantages and harms of lie which were already mentioned, are much less and do not overweigh its advantage and benefit.
It is also noteworthy that prescription of "lie" in such cases is just like prescription of eating carrion, which should be contented as much as the necessity, and not so that exceptions cause daring performance of this "capital sin", and saying lie for each little issue with the pretext of lie permits.
In fact, moral mistakes are always around the exceptional cases and notes. These are the same ambiguous matters which are the "risk area" bounded by the "forbidden area": The forbidden degrees are divine forbidden areas; one who leads his animal near the forbidden boundaries, there is the risk of slumping in them.(15) Ghazali at the end of a chapter under the title of leaving lie in "Ehyaol Oloum" writes: some people believe that fabrication of traditions in the virtues of actions and intensification of vices is permitted. Then he adds: this is just a false desire and fancy, because its advantages never overweigh corruption of lie. Moreover, there is not any necessity for this, and there are enough correct verses and traditions in each respect. On the other hand, introducing this custom may result in discomposure and perturbation of religion.(16)
But we believe such foolish actions and ideas belong to the base, fool and unaware individuals who assume themselves more compassionate than God and Prophet S.A. for Islam. This is a pure evil and is thoroughly harm and corruption. The danger of this action and such like fool friends is not less than the danger of enemies for Islam.

What is equivocation?
Equivocation refers to the speech, from the appearance of which something is understood, while the purpose of speaker is something else. It is well known among our jurists that the cases in which lie is prescribed for an exigency, equivocation should be applied, and as far as equivocation is possible, explicit lie is not advisable.
It is inferred from some phrases of Sunnite scholars that the same is popular among them. And equivocation is defined to be when a man says something with a meaning in accordance with the reality, but the speech is so that listener understands something else, and this is the purpose of speaker that while willing a correct concept, the listener understands something else. For instance, one was asked, who the substitute of Prophet S.A is. He said, "One whose daughter was in his house". The listener supposed that he meant the one whose daughter was in Prophet's S.A. house, but he meant "one who Prophet's daughter was in his house".

A new interpretation about equivocation
But herein there is a point which may be the key of solving problems of this discussion, that is, "equivocation" does not refer to any sort of intention against appearance. Rather, equivocation appertains to the cases in which the speech is inherently ambiguous and can bear two meanings, but the mind attracts one of them in speech. For example, the phrase (اِنَّكُمْ لَسارِقُونَ), "You are robbers" which was told to Joseph brothers, its primary meaning is the same "current robbery", but it can also refer to the "previous robbery" (robbing Joseph from his father), which is in contrary with its primary meaning.
Or when it is asked from someone, "has that person presented this clothing to you"? In reply as equivocation, he says: "God may lengthen his life". The listener supposes that the speaker means "yes, God may lengthen his life", while he may not have such purpose.

References:
1- Majlessi, Beharol Anvar, 8/71
2- Mohammad sura (47), verse 30
3- Tabarsi, Meshkatol Anvar, 300, Heydarieh Publications, Najaf
4- The same document, page 299
5- Majlessi, Beharol Anvar, 263/72
6- Repentance sura (9), verse 77
7- The Bee sura (16), verse 105
8- Majlessi, Beharol Anvar, 262/72
9- Majlessi, Beharol Anvar, 249/72
10- The same document, volume 78, page 9
11- Nahjol Balaghe, aphorisms, no. 38
12- Women sura (4), verse 69
13- Horre Ameli, Vassaelo Shiite, 577/8
14- Horre Ameli, Vassaelo Shiite, 134/16
15- Sadough, Faghih, vol. 4, page 75
16- Ghazali, Ehyaol Oloum, 139/3