Excerpted from Wolfgang's Kohler's book The Mentality of Apes (1925).
Two sets of interests lead us to test the intelligence of the higher apes. We are aware that it is a question of beings which in many ways are nearer to man than to the other ape species; in particular it has been shown that the chemistry of their bodies, in so far as it may be perceived in the quality of the blood, and the structure of their most highly-developed organ, the brain, are more closely related to the chemistry of the human body and human brain-structure than to the chemical nature of the lower apes and their brain development. These being show so many human traits in their "everyday" behavior that the question naturally arises whether they do not behave with intelligence and insight under conditions which require such behaviour. This question expresses the first, one might say, na´ve, interest in the intellectual capacity of animals. We wished to ascertain the dergree of relationship between anthropoid apes and man in a field which seems to us particularly important, but on which we have as yet little information.
The second aim is theoretical. Even assuming the anthropoid ape behaves intelligently in the sense in which the word is applied to man, there is yet from the very start no doubt that he remains in this respect far behind man, becoming perplexed and making mistakes in relatively simple situations; but it is precisely for this reason that we may, under the simplest conditions, gain knowledge of the nature of intelligent acts. The human adult seldom performs for the first time in his life tasks involving intelligence of so simple a nature that they can be easily investigated; and when in more complicated tasks adult men really find a solution, they can only with difficulty observe their own procedure. So one may be allowed the expectation that in the intelligent performances of anthropoid apes we may see once more in their plastic state processes with which we can no longer immediately recognize their original form: but which, because of their very simplicity, we should treat as the logical starting-point of theoretical speculation.
As all the emphasis in the following investigations is laid on the first question, the doubt may be expressed whether it does not take for granted a particular solution of the problems treated under the second. One might say that the question whether intelligent behavior exists among anthropoid apes can be discussed only after recognizing the theoretical necessity of distinguishing between intelligent behavior and behavior of any kind; and that, since association psychology, in particular, claims to derive from one single principal all behavior which would come under consideration here, up to the highest level, even that attained by human beings, a theoretical point of view is already assumed by the formulation if problem I; and one which is antagonistic to association psychology.