For me, however, the transformation was unforgettable and very specific. I can remember that when I was a child and people spoke of the "brain," I had always assumed that it was just a figure of speech. It seemed obvious that my mind was not a material substance, so people couldn't really mean it when they said that we had a physical organ that was responsible for our minds in the same way that a heart was responsible for circulation and a stomach for digestion. When I had learned that there really was a brain, I just filed it away as a kind of oddity, never even tying it together with my only other early contact with the mind/body problem (one solipsistic summer).
But then Hebb reminded us of the problem anew, first through suggestive accounts of his work with Penfield on the localization of memories in the brain, and then from the viewpoint of his own specific hypothesis that thoughts could actually be the activity of reverberating circuits of neurons called "cell-assemblies." I don't think his idea had its full impact on me at the moment he described it. Rather, it was after the lecture, as I thought about it, and thought that my thoughts may well consist of those physical things I was thinking about, that I realized what a radically different world view such a theory represented, and that it all had a ring of reality to it that made the Freudian notions I had been flirting with sound like silly fairy tales. Here were the real unconscious processes underlying our thinking, instead of the anthropomorphic machinations of some Freudian "unconscious mind," which now began to look rather like a supernumerary and supererogatory alter homunculus: One mind/body problem was enough!
Then, almost before the revelation his hypothesis represented had had a chance to take effect, Hebb took it back, informing us that his theory was almost certainly wrong. What followed was his second revelation: That a theory need not be right in order to be informative and to guide us in the right direction. And the cell assembly theory (together with other ideas in Hebb's epochal 1949 monograph, The Organization of Behaviour) had indeed inspired an enormous wealth of research findings, from the effects of sensory enrichment and deprivation to electrical and chemical pleasure centers in the brain to theoretical modeling of neural networks, as we went on to learn panoramically from the rest of Psychology 21 (based, as it was, on Hebb's Textbook of Psychology, which was itself based largely on research inspired and organized by his ideas).
Hebb himself was not only altogether unpretentious but also ever sceptical about his ideas. He saw them as pointing the way toward answers, rather than representing the answers themselves. In this I believe he had had a veridical insight into the state of contemporary psychology: he did not see much that was lapidary in it. A student of Karl Lashley, whose own contribution had been mainly critical and heuristic, Hebb always stated with complete conviction that he regarded B. F. Skinner as the greatest psychologist of the century. This, despite the fact that (in my opinion) Hebb's own work and the research it provoked (rather than Chomsky's celebrated review of Skinner's Verbal Learning) may well turn out to be seen historically as having provided the real empirical alternative to behaviorism.
Contemporary cognitive science lays claim to a variety of roots (linguistic, philosophical, computational), but it also takes a lot for granted. Without any particular brief for symbolic representation, Hebb had been arguing for four decades that thoughts are processes represented in the head, and that behaviorism, in an over-reaction against introspectionism, was begging the important questions in psychology. For Hebb, the business of scientific psychology was to make inferences about the unobservable physical substrates of behavior, thinking, personality and emotion. Such inferences were risky (unlike behaviorism's reinforcement schedules), in that they might be very wrong; but as long as they suggested testable predictions and successfully guided research, they were leading toward the truth about the mind. Another negative lesson Hebb had learned from behaviorism was that it is unwise scientific practice to ignore anything, be it our brain, our biological heritage, our cognition or our conscious experience. There is the room---indeed the need---in Hebb's cognitive psychobiology for studying all of these. And just as he avoided arbitrary dismissiveness and question-begging, Hebb resisted any sense of premature closure, of already having found the answers. (Contemporary cognitive science might do well to introspect somewhat as to whether, in its own animistic interpretations of symbolic processes, it has not inadvertently rejoined the hermeneutic road to Vienna.)
Hebb may well have been taking the proper measure of his subject when he declined the honor of having been its greatest contributor to date. In my view, his tribute to Skinner expressed his conviction that whereas theoretical ideas will come and go for some time to come in scientific psychology, its methodological commitment to ultimate behavioral testability (which is to say, empirical testability) is here to stay.
I went on to do honors psychology at McGill, and to write my honors thesis under Hebb (the last thesis he supervised, to my knowledge), and I continue to regard everything I do in psychology as Hebb-inspired. Many people feel this way about their own personal intellectual debt to Hebb (although "debt" is not really the right word---it's much more like "credit"), especially McGill people. There is also a related tendency on the part of the students of "D. O." (as Hebb was affectionately known locally) to see all good things in psychology as having issued from him. Perhaps this is a weakness of the students of all great teachers, and D. O. would certainly have been the first to pooh-pooh it. But before I set it down to mere discipial bias, I shall await the judgment of objective historians.
Historians have been relatively silent about D.O. so far, partly, no doubt, out of deference for the fact that he was still alive. For his part, D.O. had, in his wisdom, been electing to keep his own counsel in his last years, declining the many invitations to pronounce on the past, present and future of psychology that inevitably accrue to the grand old men of a field. He abstained in part because he remained ever the shrewd and dispassionate observer of life-cycle effects in his cognitive capacities (and those of others). And in part it was out of the perennial modesty and scepticism of this down-to-earth and very humane man.
But now that the old mariner's body has completed its last voyage, it is time we evaluated in earnest the treasures left us by his mind.
Stevan Harnad, Princeton, 1985