Psychological theories of learning employed in mathematics education separate intellect and affect. As a result, if affect (emotions) enters investigations of mathematical thinking and understanding at all, it is considered as an outside force or condition that generally diminishes cognition. This, however, is a hidden form of Cartesianism that separates the ideal-mental from the sensual-bodily aspects of being in the world. A very different approach was proposed in the notes of the Russian psychologist L. S. Vygotsky near his death. In the Spinozist-Marxian take of these writings, we find a “unity/identity of intellectual and affective processes” no longer “divorced from the full vitality of life, from the motives, interests, and inclinations of the thinking individual” (Vygotsky LS. The collected works of L. S. Vygotsky, vol. 1: Problems of general psychology. Springer, New York, 1987, p. 50). Educators therefore can foster mathematical thinking and understanding only when they address not only the intellectual side, but affect in intellect. In this chapter, I start with the seeds of the Spinozist-Marxian take found in Vygotsky’s last writings and develop them into a post-constructivist account of affect in intellect, which constitutes the foundation of an approach that cultures affect—in the senses of cultivating and making affect a cultural feature—and thereby leads to affective cultures of mathematics education that inherently foster thinking and understanding.