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Wherein Reasoning consists.
      §2. If general Knowledge, as has been shewn, consists in a Per-
ception of the Agreement, or Disagreement of our own Ideas; and
the Knowledge of the Existence of all Things without us (except
only of a GOD whose existence every Man may certainly know and
demonstrate to himself from his own existence) be had only by our
Senses; What room then is there for the Exercise of any other
Faculty, but outward Sense and inward Perception? What need is
there of Reason? Very much; both for the enlargement of our Know-
ledge, and regulating our Assent: For it hath to do, both in Know-
ledge and Opinion, and is necessary, and assisting to all our other
intellectual Faculties, and indeed contains two of them, viz.
Sagacity and Illation. By the one, it finds out, and by the other, it so
orders the intermediate Ideas, as to discover what connexion there is
in each link of the Chain, whereby the Extremes are held together;
and thereby, as it were, to draw into view the Truth sought for,
— 669 —
which is that we call Illation or Inference, and consists in nothing but
the Perception of the connexion there is between the Ideas, in each
step of the deduction, whereby the Mind comes to see, either the
certain Agreement or Disagreement of any two Ideas, as in Demon-
stration, in which it arrives at Knowledge; or their probable con-
nexion, on which it gives or with-holds its Assent, as in Opinion.
Sense and Intuition reach but a very little way. The greatest part
of our Knowledge depends upon Deductions and intermediate
Ideas: And in those Cases, where we are fain to substitute Assent
instead of Knowledge, and take Propositions for true, without being
certain they are so, we have need to find out, examine, and compare
the grounds of their Probability. In both these Cases, the Faculty
which finds out the Means, and rightly applies them to discover
Certainty in the one, and Probability in the other, is that which we
call Reason. For as Reason perceives the necessary, and indubitable
connexion of all the Ideas or Proofs one to another, in each step of any
Demonstration that produces Knowledge: so it likewise perceives the
probable connexion of all the Ideas or Proofs one to another, in every
step of a Discourse, to which it will think Assent due. This is the
lowest degree of that, which can be truly called Reason. For where
the Mind does not perceive this probable connexion; where it does
not discern, whether there be any such connexion, or no, there
Men’s Opinions are not the product of Judgment, or the Con-
sequence of Reason; but the effects of Chance and Hazard, of a
Mind floating at all Adventures, without choice, and without
direction.
Locke Hum IV, 17, §2, pp. 668-669