One might describe Valéry, in the terminology of recent English philosophical history, as searching unsuccessfully for a naturalistic concept of reason,1 as trying to track down, capture and display it. A philosopher who produced a stimulatingly naturalistic form of rationalism is André Lalande.2 His doctoral thesis of 1899 was entitled L’Idée directrice de la dissolution, opposée à celle de l’évolution, dans la méthode des sciences physiques et morales. This was republished in a revised form in 1930 under the title Les Illusions évolutionnistes, and is an attack on the wide nineteenth-century application of evolutionary ideas. The idea of progress is by definition the idea of movement in a desired direction, and with the tendency to confuse progress (or desirable change) and evolution (or differentiation), the latter, a fundamentally scientific notion, of limited applicability, came to acquire a normative colouring. Lalande points to a duality observable in things. Opposed to, and even more widespread than, evolution is dissolution, or, as he prefers to call it in his book of 1930, involution, since dissolution inevitably suggests some kind of degeneracy, and since it is as something desirable that he wishes to present the opposite of evolutionary change. His theory has ethical implications which are highly naturalistic. Involution is change in a certain direction and is widely exemplified in nature; reason brings about involution in the sphere of human affairs, and this is desirable and good.