AbstractHebb claimed that Lashley, over the final two decades of his career, focussed exclusively on pointing out deficiencies in existing theory and stopped making positive contributions of his own. This characterization, which Hebb also suggested why Lashley declined to collaborate on The Organization of (D. O. Hebb, 1949), is shown to be an overstatement. Lashley's coolness from the very outset toward Hebb's neuropsychological theory probably stemmed from disagreement with its substance. Yet it a theory that in the end, Lashley appeared to accept. Curiously, Hebb's complaint about Lashley's excessively critical bent coexisted with a more measured view of his theoretical approach, a view in keeping with the one presented in this essay.In his day, Karl Lashley's critical tendencies were well - known. Beach (1961) described him a theorist who specialized in disproving theories, including his own (p. 163). Orbach (1982), Lashley's last post - doctoral student, observed that as a critic he a demolition expert (p. 43). Hebb, who took his Ph.D degree with Lashley, frequently noted the negative impact of his research (e.g., Hebb, 1963). In his autobiography, however, Hebb (1980) took a more extreme stance, averring that Lashley's interest in theory entirely negative and critical: He was so interested in criticizing others' theories and so little interested in developing one himself (p. 297). Although this the first time that Hebb had publicly aired that view, it had had a long gestation period. Some 25 years earlier, during the editing of the volume of Lashley's articles (Beach, Hebb, Morgan, & Nissen, 1960), Hebb remarked to his co - editors, think also that it about 1937 when he [Lashley] really gave up any attempt at constructive, opposed to critical, contributions ... (letter to F. Beach, November 23, 1955).In this article, I take up the criticisms and connections of the two protagonists pictured in Figure 1, Karl Lashley (1890 - 1958) and Donald Hebb (1904 - 1985). The story may be appreciated at any number of levels: an example of the tension and competitiveness that sometimes develops between mentor and student; a depiction of the character and motivation of two very famous psychologists; part of the intellectual and social backdrop of a landmark event in the history of neuropsychology - the publication in 1949 of Hebb's The Organization of Behavior, and a detective story in which I solve, or at least unearth evidence bearing on, some historical puzzles, minor ones perhaps but no less intriguing on that account.By way of a preview of my account, I begin with an evaluation of the validity of Hebb's statement that Lashley's post - 1937 interest in theory exclusively critical - Hebb's criticism I will occasionally call it - by examining Lashley's articles published after that date. Significant exceptions to Hebb's claim are easy to come by, but I limit myself to a description of three of Lashley's positive contributions and passing reference to a fourth. This will suffice to allay any concern that even the spirit of Hebb's criticism is correct - that in the main Lashley a critic. Having shown that Hebb's assessment is overdrawn, I then ask why he ventured it in the first place. The reason appears to be the short shrift that Lashley gave to Hebb's (1949) otherwise well received book, The Organization of Behavior. Why Lashley all but ignored the theory that Hebb proposed there is more opaque, but I will hazard a solution to that curious puzzle well. My speculation, and some additional conjectures, will be based on a consideration of Lashley's unpublished 1952 Vanuxem lectures, his final published article, Cerebral Organization and Behavior (Lashley, 1958), and correspondence of Lashley and of Hebb.LASHLEY'S THEORETICAL CONTRIBUTIONS AFTER 1937The Problem of Serial Order in Behavior (Lashley, 1951). …