Inhuman Thoughts is a philosophical exploration of the possibility of increasing the physiological and psychological capacities of humans to the point that they are no longer biologically, psychologically, or socially human. The movement is from the human through the trans-human, to the post-human. The tone is optimistic; Seidel argues that such an evolution would be of positive value on the whole.

Seidel's initial argument supports the need for a comprehensive ethical theory, the success of which would parallel that of a large-scale scientific revolution, such as Newtonian mechanics. He elaborates the movement from the improved-but-still-human to the post-human, and philosophically examines speculated examples of post-human forms of life, including indefinitely extended life-span, parallel consciousness, altered perception, a-sociality, and a-sexuality.

Inhuman Thoughts is directed at those interested in philosophical questions on human nature and the best life given the possibilities of that nature. Seidel's overall argument is that the most satisfactory answer to the latter question involves a transcendence of the present confines of human nature.