CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together), a stipulation, agreement. The term is applied technically to any circumstance, action or event which is regarded as the indispensable prerequisite of some other circumstance, action or event. It is also applied generally to the sum of the circumstances in which a person is situated, and more specifically to favourable or prosperous circumstances; thus a person of wealth or birth is described as a person "of condition," or an athlete as being "in condition," i.e. physically fit, having gone through the necessary course of preliminary training. In all these senses there is implicit the idea of limitation or restraint imposed with a view to the attainment of a particular end.
(1) In Logic, the term "condition" is closely related to "cause" in so far as it is applied to prior events, etc., in the absence of which another event would not take place. It is, however, different from "cause" inasmuch as it has a predominantly negative or passive significance. Hence the adjective "conditional" is applied to propositions in which the truth of the main statement is made to depend on the truth of another; these propositions are distinguished from categorical propositions, which simply state a fact, as being "composed of two categorical propositions united by a conjunction," e.g. if A is B, C is D. The second statement (the "consequent") is restricted or qualified by the first (the "antecedent"). By some logicians these propositions are classified as (1) Hypothetical, and (2) Disjunctive, and their function in syllogistic reasoning gives rise to the following classification of conditional arguments: - (a) Constructive hypothetical syllogism (modus ponens, "affirmative mood"): If A is B, C is D; but A is B; therefore C is D. (b) Destructive hypothetical syllogism (modus tollens, mood which "removes," i.e. the consequent): if A is B, C is D; but C is not D; therefore A is not B. In (a) the antecedent must be affirmed, in (b) the consequent must be denied; otherwise the arguments become fallacious. A second class of conditional arguments are disjunctive syllogisms consisting of (c) the modus ponendo tollens: A is either B or C; but A is B; therefore C is not D; and (d) modus tollendo ponens: A is either B or C; A is not B; therefore A is C. A more complicated conditional argument is the dilemma (q.v.). 
The limiting or restrictive significance of "condition" has led to its use in metaphysical theory in contradistinction to the conception of absolute being, the aseitas of the Schoolmen. Thus all finite things exist in certain relations not only to all other things but also to thought; in other words, all finite existence is "conditioned." Hence Sir Wm. Hamilton speaks of the "philosophy of the unconditioned," i.e. of thought in distinction to things which are determined by thought in relation to other things. An analogous distinction is made (cf. H. W. B. Joseph, Introduction to Logic, pp. 380 foll.) between the so-called universal laws of nature and conditional principles, which, though they are regarded as having the force of law, are yet dependent or derivative, i.e. cannot be treated as universal truths. Such principles hold good under present conditions, but other conditions might be imagined under which they would be invalid; they hold good only as corollaries from the laws of nature under existing conditions.
(2) In Law, condition in its general sense is a restraint annexed to a thing, so that by the non-performance the party to it shall receive prejudice and loss, and by the performance commodity or advantage. Conditions may be either: (1) condition in a deed or express condition, i.e. the condition being expressed in actual words; or (2) condition in law or implied condition, i.e. where, although no condition is actually expressed, the law implies a condition. The word is also used indifferently to mean either the event upon the happening of which some estate or obligation is to begin or end, or the provision or stipulation that the estate or obligation will depend upon the happening of the event. A condition may be of several kinds: (1) a condition precedent, where, for example, an estate is granted to one for life upon condition that, if the grantee pay the grantor a certain sum on such a day, he shall have the fee simple; (2) a condition subsequent, where, for example, an estate is granted in fee upon condition that the grantee shall pay a certain sum on a certain day, or that his estate shall cease. Thus a condition precedent gets or gains, while a condition subsequent keeps and continues. A condition may also be affirmative, that is, the doing of an act; negative, the not doing of an act; restrictive, compulsory, etc. The word is also used adjectivally in the sense set out above, as in the phrases "conditional legacy," "conditional limitation," "conditional promise," etc.; that is, the legacy, the limitation, the promise is to take effect only upon the happening of a certain event.
 The terminology used above has not been adopted by all logicians. "Conditional" has been used as equivalent to "hypothetical" in the widest sense (including "disjunctive"); or narrowed down to be synonymous with "conjunctive" (the condition being there more explicit), as a subdivision of "hypothetical."
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)