The importance of biology was probably best expressed by the philosopher of religion, Abraham J. Heschel (1965). He noted that whereas "a theory about the stars never becomes part of the being of the stars", a theory about human beings enters our consciousness, determines our self-image, and modifies our very existence. "The image of man affects the nature of man ... We become what we think of ourselves." If we think of ourselves as killer apes, certain behavioral phenotypes are acceptable that would not be tolerated if we view humans as the current apex of an evolutionary trend towards cooperation.