profiting by experience.” It is a question rather of the rapidity and of the kind of learning involved. “The fact that the crayfish needs a hundred or more experiences for the learning of a type of reaction that the frog would learn with twenty experiences, the dog with five, say, and the human subject with perhaps a single experience, is indicative of the fundamental difficulty in the use of this sign.” Nagel has pointed out that Loeb, in asserting “associative memory” as the criterion of consciousness, offers no evidence for his statement. The fact is that while proof of the existence of mind can be derived from animal learning by experience only if the learning is very rapid, other evidence, equally valid on the principle of analogy, makes it highly improbable that all ani mals which learn too slowly to evince the presence of ideas are therefore unconscious. This evidence is of a morphological character.
§7. INFERRING MIND FROM STRUCTURE. Both Yerkes and Lukas urge that the resemblance of an animal's nervous system and sense-organs to those of human beings ought to be taken into consideration in deciding whether the animal is conscious or not. Lukas suggested that the criteria of consciousness should be grouped under three heads: morphological, including the structure of the brain and sense-organs, physiological, and teleological. Under the second rubric he maintained that “individual purposiveness” is characteristic of the movements from which consciousness may be inferred; that individual purposiveness pertains only to voluntary acts, and that voluntary acts are acts “which are preceded by the intention to perform a definite movement, hence by the idea of this movement.” We have reached the same conclusion in the preceding paragraph. The third test of the presence of consciousness, the teleological test, rests on the consideration: “What significance for the organism may be possessed by the production of a conscious effect by certain stimuli?” This test, however, being of a purely a priori character, would seem to be distinctly less valuable than the others.
Yerkes proposed “the following six criteria in what seems to me in general the order of increasing importance. The functional signs are of greater value as a rule than the structural; and within each of the categories the particular sign is usually of more value than the general. In certain cases, however, it might be maintained that neural specialization is of greater importance than modifiability.
“I. Structural Criteria.
1. General form of organism (Organization).
2. Nervous system (Neural organization).
3. Specialization in the nervous system (Neural
II. Functional Criteria.
1. General form of reaction (Discrimination).
2. Modifiability of reaction (Docility).
3. Variability of reaction (Initiative).”
The terms “discrimination,” “docility,” and “initiative” in this connection are borrowed from Royce's “Outlines of Psychology.”
If resemblance of nervous and sense-organ structure to the human type is to be taken along with rapid learning as co-ordinate evidence of consciousness, it is clear that here also we have to deal with a matter of degree. The structure of the lower animals differs increasingly from our own as we go down the scale. At what degree of difference shall we draw the line and say that the animals above it may be conscious, but that those below it cannot be? No one could possibly establish such a line. The truth of the whole matter seems to be this: We can say neither what amount of resemblance in structure to human beings, nor what speed of learning, constitutes a definite mark distinguishing animals with minds from those without minds, unless we are prepared to assert that only animals which learn so fast that they must have memory ideas possess mind at all. And this would conflict with the argument from structure. For example, there was until recently no good experimental evidence that cats possess ideas, yet there was enough analogy between their nervous systems and our own to make it improbable that consciousness, so complex and highly developed in us, was in them wholly lacking. We know not where consciousness begins in the animal world. We know where it surely resides—in ourselves; we know where it exists beyond a reasonable doubt—in those animals of structure resembling ours which rapidly adapt themselves to the lessons of experience. Beyond this point, for all we know, it may exist in simpler and simpler forms until we reach the very lowest of living beings.