The seventeenth-century polymath Guarino Guarini is today mainly remembered as one of the most prominent Baroque architects. However, he was also active in a number of other fields such as philosophy, mathematics and astronomy. Philosophically, Guarini is an exceptionally interesting figure because of his efforts to adjust his Counter-reformation Aristotelianism to the latest discoveries of the ongoing scientific revolution. While his philosophy belonged to so-called “second scholasticism,” unlike most other scholastic philosophers of the era he followed and was intensely interested in the latest scientific breakthroughs. The book analyses how his philosophical and scientific interests impacted his views on architectural theory and architectural design.