Reasoning, n.
1. The act or process of adducing a reason or reasons; manner of presenting one's reasons.
2. That which is offered in argument; proofs or reasons when arranged and developed; course of argument. His reasoning was sufficiently profound. Macaulay.
Syn. -- Argumentation; argument. -- Reasoning, Argumentation. Few words are more interchanged than these; and yet, technically, there is a difference between them. Reasoning is the broader term, including both deduction and induction. Argumentation denotes simply the former, and descends from the whole to some included part; while reasoning embraces also the latter, and ascends from a part to a whole. See Induction. Reasoning is occupied with ideas and their relations; argumentation has to do with the forms of logic. A thesis is set down: you attack, I defend it; you insist, I prove; you distinguish, I destroy your distinctions; my replies balance or overturn your objections. Such is argumentation. It supposes that there are two sides, and that both agree to the same rules. Reasoning, on the other hand, is often a natural process, by which we form, from the general analogy of nature, or special presumptions in the case, conclusions which have greater or less degrees of force, and which may be strengthened or weakened by subsequent experience.
Shades of meaning
Argumentation and debate, in the ordinary use of the words, suppose two parties alleging reasons for and against a proposition; the same idea appears figuratively when we speak of a debate or an argument with oneself, or of a debate between reason and conscience. Reasoning may be the act of one alone, as it is simply the orderly setting forth of reasons, whether for the instruction of inquirers, the confuting of opponents, or the clear establishment of truth for oneself. Reasoning may be either deductive or inductive. Argument or argumentation was formerly used of deductive reasoning only. With the rise of the inductive philosophy these words have come to be applied to inductive processes also; but while reasoning may be informal or even (as far as tracing its processes is concerned) unconscious, argument and argumentation strictly imply logical form. Reasoning, as denoting a process, is a broader term than reason or argument; many arguments or reasons may be included in a single chain of reasoning.