If a frog, on the flank of which a little drop of acid has been placed, rubs it off with the foot of the same side; and, if that foot be held, performs the same operation, at the cost of much effort, with the other foot, it certainly displays a curious instinct.

But it is no less true that the whole operation is a reflex operation of the spinal cord, which can be performed quite as well when the brain is destroyed; and between which and simple reflex actions there is a complete series of gradations.

In like manner, when an infant takes the breast, it is impossible to say whether the action should be rather termed instinctive or reflex.
What are usually called the instincts of animals are, however, acts of such a nature that, if they were performed by men, they would involve the generation of a series of ideas and of inferences from them; and it is a curious, and apparently an insoluble, problem whether they are, or are not, accompanied by cerebral changes of the same nature as those which give rise to ideas and inferences in ourselves.

When a chicken picks up a grain, for example, are there, firstly, certain sensations, accompanied by the feeling of relation between the grain and its own body; secondly, a desire of the grain; thirdly, a volition to seize it?
Or, are only the sensational terms of the series actually represented in consciousness?
The latter seems the more probable opinion, though it must be admitted that the other alternative is possible.

But, in this case, the series of mental states which occurs is such as would be represented in language by a series of propositions, and would afford proof positive of the existence of innate ideas, in the Cartesian sense.