The response to painful conditioning in the simplest forms of life.

"Take the simplest forms of physical life, for instance and bring them repeatedly into contact with the same type of stimuli. It has been found that as a result of experience, they react more quickly to them after a time than they did at first.

A single-celled organism with the power of locomotion will retreat from a drop of injurious acid more quickly after it has had some previous experience in close approach to such an acid."

Or in a more complex form.

"And a baby, after being roughly handled a time or two by a careless adult, will more quickly cry and thrash about at the approach of this adult, in the endeavour to escape a similar hardship, than it did before the rough handling.

The response to pleasurable conditioning in the simplest forms of life.

"Furthermore, a single-celled organism when brought in contact with an object which affords satisfactory food, after repeated experience, acts more quickly and effectively to avail itself of the food that it did on the first occasion of its contact.

The response to pleasurable conditioning in a more complex form of life.

And a human infant having been fed or petted by an adult will crow and reach out its arms toward this adult in a manner which it did not do before."