This lecture introduces the towering figure of Leon Battista Alberti, the embodiment of the Renaissance "polymath" who redefined architecture from a manual trade to a high intellectual discipline. We will explore the birth of the "theoretical architect," examining how Alberti revived the lost wisdom of the Roman writer Vitruvius to champion a built environment governed by musical harmony and mathematical ratio rather than mere intuition. We will see these abstract concepts realised in stone through two key Florentine projects: the completion of the Santa Maria Novella façade, where he used ingenious geometry to resolve existing Gothic irregularities, and the Palazzo Rucellai. By analysing the Palazzo’s flat, elegant pilasters—superimposed in the manner of the Colosseum—we will define how Alberti introduced a new, literate classicism that challenged the fortress-like traditions of the city.