Man was simply the result of the interaction of numerous causes--and like the rest of nature, involved no independent spiritual element.

Everything that was previously regarded as spiritual was interpreted as a mere adjunct to, or a shadow of, the sense world.
Such a conception accounted for the whole of nature and of man, and so became an explanation of the universe, a philosophy.
In such a theory self-preservation becomes the aim of life, the struggle for existence the driving-power, and adaptation to environment the means to the desired end.

Hence it comes about that only one standard of value remains, that of usefulness, for that alone can be regarded as valuable which proves to be useful towards the preservation and enjoyment of the natural life.

The ideas of the good, the beautiful, and the true, lose the glory of their original meaning, and become comparatively barren conceptions.

Hence at a stroke the spiritual world is wiped away, the soul of man is degraded from its high position, the great truths of religion are cast aside as mere illusions.
The naturalistic explanation possesses the apparent advantage of being a very simple one, and hence attracts the human mind with great force in the early stages of mental culture.