The third and fourth books give evidence of acuteness in psychological analysis; the fourth and sixth of the most active and varied observation of natural phenomena; the fifth of original insight and strong common sense in conceiving the origin of society and the progressive advance of man to civilization.
This has been criticized both from the biological and from the psychological standpoint.
27, has become a psychological symbol.
From the biological point of view the reference of certain modes of behaviour, termed instinctive, to faculties of mind for which "instinct" is the generic term is scarcely satisfactory; from the psychological point of view the phrase "without necessary knowledge of the relation between the means employed and the end attained" is ambiguous.
He then gave himself up to philosophical work, especially in connexion with the phenomena of hypnotism and occultism from the modern psychological standpoint.