This conceptual work is guided by logical and aesthetic criteria, by elegance and simplicity. By Kant’s own testimony, the contemplation of nature was for him an intensely aesthetic encounter. In the Observations on the Feeling of the Beautiful and the Sublime (1764), his greatest public success before the Critiques, he would develop a phenomenology of beauty. Depending on the intensity of the experience, an aesthetic encounter can be either with beauty, exciting pleasure—or with the sublime, striking awe, sensual overload, even terror (2:208–9). Later, in the Critique of Judgment (1790), he would argue for the heuristic value of aesthetic notions such as design and unity.