This book's message seems rather straightforward, stimulating, and more or less plausible, but the development and presentation of its core ideas are somewhat fuzzy. For starters, the term "rationality" is not explicitly defined, and the sense in which this highly overloaded word is employed here is left to the reader to infer. My take is that the author's usage refers to a "procedural" rationality that observes the norms of the scientific method, and in consequence applies rigorous methods in theory formulation. In any case, however, it is not clear how rationality could necessarily exclude or inhibit reasonableness, except in obtuse practice.
On page 203, it is stated that "the recovery of Reasonableness can restore the concept of Rationality to the richness" deprived it by Descartes. So reasonableness can temper the exercise and efficacy of rationality, as through some judicious combination thereof. This is essentially what I take Holton's thematic dimension to portray - in actual, historical scientific practice (see Chapter 1 of "Thematic Origins of Scientific Thought"). This dimension includes "fundamental presuppositions, notions, terms, methodological judgments, and decisions". These are all unverifiable and unfalsifiable elements, in contrast with the content of Holton's analytic-empirical plane that is orthogonal to the thematic dimension. In any case, the Enlightenment's legacy has of course included the exaggeration of the capabilities of rationality. Or more apropos here, their uncritical or inappropriate deployment has clearly resulted in such excesses or failures as Toulmin regrets.
Another case of message clouding pertains to the tension between reasonableness and rationality that appears on page 2. Here these two concepts are purported to be complementary, i.e., polar opposites. Instead, the opposite of rationality would seem to be spontaneity, impulsivity, or extemporization. Reasonableness would then lie along the continuum between rationality (rigorous rule following) and spontaneity (intuitive redirection), in the form of rationality augmented by spontaneity. Such spontaneity would yield the often-decisive bursts of opportunistic insight that propel scientific advances. This continuum is analogous to Hammond's analytical-intuitive continuum. Moreover, his term quasirationality is a very apt synonym for generic reasonableness (see Chapter 6 of "Human Judgment and Social Policy").
Alas, my above of construal of reasonableness is apparently not what Toulmin had in mind. I see reasonableness as in fact largely augmenting the rigorous scientific effort associated with Holton's aforementioned analytic-empirical plane. Research legitimately associated with this plane itself constitutes idealized hard science, and efforts in the thematic-axis are facilitative scaffolding that embodies reasonableness. In contrast, after Toulmin characterizes clinical practice, historical case studies, and action research, etc., he seems to equate such efforts with reasonableness itself. Moreover, in his explication, this variety of reasonableness displaces rather than augments rationality. Such practices are undoubtedly efficacious in many cases, but these sorts of activities hardly constitute or resemble science.
Furthermore, the author's rejection of Popper et al.'s contention that the theory of evolution is not bone fide science appears to rest on the author's implication that reasonableness of narrative or argument per se may stand as science. However, any such resultant theory is inherently speculative, unverifiable, and unfalsifiable. To designate such tenuous or lightweight caliber of theory as science is "unreasonable". That would call for a rather loose definition of the term science, or else a substantial qualification of the author's usage, to render the term science so broadly inclusive.
Beyond these not insignificant reservations, I found "Return to Reason" to be a stimulating and in parts an enlightening read. The author's tracing of the trajectory of reasoning and its attendant predispositions from antiquity through modernity was quite illuminating, especially with regard to the post-Enlightenment divergence into an inordinate fixation on a strict or compulsive version of rationality. As this book is the fourth by Toulmin that I have read, it happens that I do admire his original and provocative thinking. As the following commentary reveals, certain of the ideas and conclusions presented in the subject book rather strongly resonate with me.
For example, for several centuries now, rationality has been an obsession if not a fetish in attempts to confer scientific authority or credibility across all disciplines, especially with respect to aspiring human sciences. Moreover, the success of physicists in achieving dramatic if misinterpreted advances since the Enlightenment has induced other disciplines to emulate their methods in rather pretentious, naive, or self-conscious ways. Such indiscriminate emulation has yielded generally disappointing results, but fortunately this fad now seems to be in substantial decline. Toulmin's generally applicable prescription is that each discipline should apply methods suited to its own nature and to the particular challenge at hand. That advice has hardly been disregarded altogether in the past by the hard sciences.
Despite my stated reservations, this is a quite worthwhile book, with a lot of valuable coverage on supporting topics like rhetoric versus logic, substantive versus theoretical argument, abstraction versus situatedness, undue generalization, and clinical practice. Toulmin offers us much here upon which to reflect, as well as major points with which to disagree perhaps, i.e., genuine intellectual stimulation.