"My philosophic studies have been somewhat restricted to the logic of mathematical physics and applied ethics, and on epistemologic problems I can speak only with an innocence which I trust may not be regarded as too childish." (The Distinction between the Mental and the Physical," (Studies in Philosophy and Science, New York, Henry Holt, p. 90). This self-description understates Cohen's achievements. He was a systematic philosopher who moved easily through detailed analyses of science, philosophy, history, law, and social policy. He believed that nature has a decided form, and that scientific method-competing, testable hypotheses-is the only effective way to study it. No hypothesis is more than probable, but truth is our aim. Cohen's realism was usually emphatic: The existence and character of things are independent of our ways of knowing them. Logic and sufficient reason (hence causality) are intrinsic to nature, where they constrain everything that is or can be; nature is replete with possibilities, most unrealized. His principle of polarity affirms that there is "necessary opposition in all determinate effects.[E]very static, but also every kinetic, system involves a balance or equilibrium which makes description in the form of equations applicable." (Studies in Philosophy and Science, p. 12).

Cohen's ethics expressed his belief that humans flourish best in a rational order that mitigates conflict and maximizes opportunities for self-expression. Natural law is an immanent constraint and an ideal: it limits and promotes conditions for well-being. Positive laws appropriate to this constraint are formulated after empirical investigations that identify human wants and aspirations. But, "No doctrine of natural law can claim a greater degree of certainty and completeness than attaches to the basic ethical principles which it presupposes." (Reason and Nature, London, Free Press of Glencoe, 1964, p. 414) Cohen located these ethical principles in the ideal of justice. Yet, there is a mismatch between this ideal and the means for knowing and achieving it. Altruism turns pragmatic when the ideal evades us. For there is no accord about justice: is it local to a time and place, or universal? Law fills the breach: "[L]aw must be, as it is, in large part a special technique for determining what would otherwise be uncertain and subject to conflict. It is socially necessary to have a rule of the road but it is morally indifferent whether it requires us to turn to the right or to the left." (Reason and Nature, p. 420)

The assertion that the mental and the physical are complexes of neutral entities may suggest the question, Where and when do these neutral entities exist, if not in the mind or in physical space. The answer is that anything may be said to exist in a given universe of discourse if it can be shown that it occupies a position therein.This may seem to degrade the term existence, and perhaps it does. But I believe that few habits would be more useful to philosophy than the habit of refusing to discuss whether certain entities exist, unless we ask exist how? Or in what kind of system" (Studies in Philosophy and Science, p. 97) This anticipation of Carnap and Quine, written in 1917, was anomalous, but not an aberration. For Cohen's realism was also compromised in this other way. A reader once questioned his exposition of special relativity: are things shortened in the direction of motion, or do they merely appear shortened? Cohen responded by taking cover in Einstein's operationalism. (Studies in Philosophy and Science, pp. 241-242). Cohen expressed these competing emphases while frequently citing Aristotle's distinction between that which is first in knowledge and that which is first in being. This could have been entrée to a comprehensive metaphysics of nature, or, more modestly, a sharp distinction between the priorities of knowledge or expression, and the character of things themselves. But Cohen hesitated.

A famous critic-more than a hundred of his papers appraised other thinkers-Cohen had no taste for cleverness, paradox, or mere technical virtuosity. Peirce was his soul-mate, Spinoza his inspiration. Cohen cited him when responding indirectly to the claim that Cohen, himself, said little that was new: "Spinoza never valued ideas for their novelty, and had no hankering to be the founder of a new system of philosophy." (The Faith of a Liberal, p. 13) But consider: Cohen was the belligerent opponent to all the truisms of his time: intuitionism, not hypothesis, was thought to be the style appropriate to all knowledge; the external world was said to have no thinkable properties; logic was thought to be a syntactic calculus; mind and body were said to be categorially different; justifications for morality were transcendental, theological, or emotivist, without foundation in the details of human character and struggle. Was it originality, or mere courage, to affirm that "logic is the simplest chapter in ontology," (Studies in Science and Philosophy, p. 150), that sufficient reason and causality are intrinsic to natural processes, that truth is correspondence, that mind is very likely a function of matter, that humans are remarkable among animals for being perfectible though justice would require our perfection.

Forgetting dead writers may be either of two things: an appraisal of their ideas, or evidence of a change in the sensibility of those who forget. No philosopher of the last hundred years exceeded Cohen's courage, depth, or erudition. Reading him alters us.