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Justice in a person and justice in an action.
The names of just and unjust when they are attributed to men, signify one thing, and when they are attributed to actions, another. When they are attributed to men, they signify conformity, or inconformity of manners, to reason. But when they are attributed to action they signify the conformity, or inconformity to reason, not of manners, or manner of life, but of particular actions. A just man therefore is he that taketh all the care he can that his actions may be all just; and an unjust man is he that neglecteth it.
Hobbes, Lev XV 10
Definition and argument
To be just is a law of nature, and consists of performance of covenant (keeping one’s agreements).
FROM that law of nature by which we are obliged to transfer to another such rights as, being retained, hinder the peace of mankind, there followeth a third; which is this: that men perform their covenants made; without which covenants are in vain, and but empty words; and the right of all men to all things remaining, we are still in the condition of war.
And in this law of nature consisteth the fountain and original of justice. For where no covenant hath preceded, there hath no right been transferred, and every man has right to everything and consequently, no action can be unjust. But when a covenant is made, then to break it is unjust and the definition of injustice is no other than the not performance of covenant. And whatsoever is not unjust is just.
Hobbes, Lev XV 1-2
There is no exception to this law.
Others, that allow for a law of nature the keeping of faith, do nevertheless make exception of certain persons; as heretics, and such as use not to perform their covenant to others; and this also is against reason. For if any fault of a man be sufficient to discharge our covenant made, the same ought in reason to have been sufficient to have hindered the making of it.
Hobbes, Lev XV 9
Explanation of error
Why people sometimes think that rights, justice, etc., are determined by custom.
Ignorance of the causes, and original constitution of right, equity, law, and justice, disposeth a man to make custom and example the rule of his actions; in such manner as to think that unjust which it hath been the custom to punish; and that just, of the impunity and approbation whereof they can produce an example or (as the lawyers which only use this false measure of justice barbarously call it) a precedent; like little children that have no other rule of good and evil manners but the correction they receive from their parents and masters; save that children are constant to their rule, whereas men are not so; because grown strong and stubborn, they appeal from custom to reason, and from reason to custom, as it serves their turn, receding from custom when their interest requires it, and setting themselves against reason as oft as reason is against them: which is the cause that the doctrine of right and wrong is perpetually disputed, both by the pen and the sword: whereas the doctrine of lines and figures is not so; because men care not, in that subject, what be truth, as a thing that crosses no man's ambition, profit, or lust. For I doubt not, but if it had been a thing contrary to any man's right of dominion, or to the interest of men that have dominion, that the three angles of a triangle should be equal to two angles of a square, that doctrine should have been, if not disputed, yet by the burning of all books of geometry suppressed, as far as he whom it concerned was able.
Hobbes, Lev XI 21