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Home > Catholic Encyclopedia > D > Deduction


DEDUCTION

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(Latin de ducere, to lead, draw out, derive from; especially, the function of
deriving truth from truth). The topic will be treated in two sections:

> I. As an argument or reasoning process: that kind of mediate inference by
> which from truths already known we advance to a knowledge of other truths
> necessarily implied in the former; the mental product or result of that
> process.
> II. As a method: the deductive method, by which we increase our knowledge
> through a series of such inferences.


AS AN ARGUMENT OR REASONING PROCESS

The typical expression of deductive inference is the syllogism. The essential
feature of deduction is the necessary character of the connexion between the
antecedent or premises and the consequent or conclusion. Granted the truth of
the antecedent judgments, the consequent must follow; and the firmness of our
assent to the latter is conditioned by that of our assent to the former. The
antecedent contains the ground or reason which is the motive of our assent to
the consequent; the latter, therefore, cannot have greater firmness or certainty
than the former. This relation of necessary sequence constitutes the formal
aspect of deduction. It can be realized most clearly when the argument is
expressed symbolically, either in the hypothetical form:

 1. "If anything (S) is M it is P;
 2. but this S is M;
 3. therefore this S is P",



or in the categorical form,

 1. "Whatever (S) is M is P;
 2. but this S is M;
 3. therefore this S is P".

The material aspect of the deductive argument is the truth or falsity of the
judgments which constitute it. If these be certain and evident the deduction is
called demonstration, the Aristotelian apodeixis. Since the conclusion is
necessarily implied in the premises, these must contain some abstract, general
principle, of which the conclusion is a special application; otherwise the
conclusion could not be necessarily derived from them; and all mediate
inferences must be deductive, at least in this sense, that they involve the
recognition of some universal truth and do not proceed directly from particular
to particular without the intervention of the universal.


AS A METHOD

When, starting from general principles, we advance by a series of deductive
steps to the discovery and proof of new truths, we employ the deductive or
synthetic method. But how do we become certain of those principles which form
our starting-points?

 * We may accept them on authority as, for example, Christians accept the
   deposit of Christian revelation on Divine authority and proceed to draw out
   their implications by the deductive reasoning which has shaped and moulded
   the science of theology.
 * Or we may apprehend them by intellectual intuition as self-evident, abstract
   truths concerning the nature of thought, of being, of matter, of quantity,
   number, etc., and thence proceed to build up the deductive sciences of logic,
   metaphysics, mathematics, etc. Down through the Middle Ages enlightened
   thought was fixed almost exclusively on those two groups of data, both sacred
   and profane; and that accounts for the fulness of the scholastic development
   of deduction.
 * But besides being and quantity, the universe presents change, evolution,
   regular recurrences or repetition of particular facts, from the careful
   observation and analysis of which we may ascend to the discovery of a third
   great class of general truths or laws.

This ascent from the particular to the general is called induction, or the
inductive or analytic method. Comparatively little attention was paid to this
method during the Middle Ages. Apparatus for the accurate observation and exact
measurement of natural phenomena was needed to give the first real impetus to
the cultivation of the physical, natural, or inductive sciences. In these
departments of research the mind approaches reality from the side of the
concrete and particular and ascends to the abstract and general, while in
deduction it descends from the general to the particular. But although the mind
moves in opposite directions in both methods, nevertheless the reasoning or
inference proper, employed in induction, is in no sense different from deductive
reasoning, for it too implies and is based on abstract, necessary truths.




ABOUT THIS PAGE

APA citation. Coffey, P. (1908). Deduction. In The Catholic Encyclopedia. New
York: Robert Appleton Company. http://www.newadvent.org/cathen/04674a.htm

MLA citation. Coffey, Peter. "Deduction." The Catholic Encyclopedia. Vol. 4. New
York: Robert Appleton Company, 1908.
<http://www.newadvent.org/cathen/04674a.htm>.

Transcription. This article was transcribed for New Advent by Rick McCarty.

Ecclesiastical approbation. Nihil Obstat. Remy Lafort, Censor. Imprimatur. +John
M. Farley, Archbishop of New York.

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