The keynote was "relation to environment"; a constantly changing environment, changing according to law, called for ceaseless readjustment, and the adaptation to environment was held to be the stimulus to all activity in the natural world.
The later development of biology, and the doctrine of the evolution of species, gradually extended this conception of nature to include man himself.
What he had regarded as his distinctive characteristics were held to be but the product of natural factors, and his life was regarded, too, as under the domain of rigid, inviolable law.

There was no room for, and no need of, the conception of free, originative thought.

Thought was simply an answer to the demand that the sense world was making, entirely dependent upon the external stimulus, just as any other activity was entirely dependent on an external stimulus.

So thought came to be regarded as resulting from mere sense impression, which latter corresponded to the external stimulus.

It is obvious that the idea of the freedom of the human soul, and of human personality as previously understood, had to go.