But our wonder will perhaps cease or diminish when we consider that the experimental reasoning itself, which we possess in common with beasts, and on which the whole conduct of life depends, is nothing but a species of instinct or mechanical power, that acts in us unknown to ourselves, and in its chief operations is not directed by any such relations or comparison of ideas as are the proper objects of our intellectual faculties.
"Though the instinct be different, yet still it is an instinct which teaches a man to avoid the fire, as much as that which teaches a bird, with such exactness, the art of incubation and the whole economy and order of its nursery."-- (IV.pp.125, 126.) The parallel here drawn between the "avoidance of a fire" by a man and the incubatory instinct of a bird is inexact.

The man avoids fire when he has had experience of the pain produced by burning; but the bird incubates the first time it lays eggs, and therefore before it has had any experience of incubation.

For the comparison to be admissible, it would be necessary that a man should avoid fire the first time he saw it, which is notoriously not the case.
The term "instinct" is very vague and ill-defined.

It is commonly employed to denote any action, or even feeling, which is not dictated by conscious reasoning, whether it is, or is not, the result of previous experience.

It is "instinct" which leads a chicken just hatched to pick up a grain of corn; parental love is said to be "instinctive"; the drowning man who catches at a straw does it "instinctively"; and the hand that accidentally touches something hot is drawn back by "instinct." Thus "instinct" is made to cover everything from a simple reflex movement, in which the organ of consciousness need not be at all implicated, up to a complex combination of acts directed towards a definite end and accompanied by intense consciousness.
But this loose employment of the term "instinct" really accords with the nature of the thing; for it is wholly impossible to draw any line of demarcation between reflex actions and instincts.