of the brain. In 1987, he became the first Visiting Scholar in the Department of Cognitive and Neural Systems (Boston University) and worked on neuroscience research for a period. Hestenes has been a principal investigator for NSF grants seeking to teach physics through modeling and to measure student understanding of physics models at both the high school and university levels. Work Hestenes has worked in mathematical and theoretical physics, geometric algebra, neural networks, and cognitive research in science education. He is the prime mover behind the contemporary resurgence of interest in geometric algebras and in other offshoots of Clifford algebras as ways of formalizing theoretical physics. Geometric algebra and calculus Spacetime algebra provided the starting point for two main lines of research: on its implications for quantum mechanics specifically and for mathematical physics generally. The first line began with the fact that reformulation of the Dirac equation in terms of spacetime algebra reveals hidden geometric structure. Among other things, it reveals that the complex factor in the equation is a geometric quantity (a bivector) identified with electron spin, where specifies the spin direction and is the spin magnitude. The implications of this insight have been studied in a long series of papers with the most significant conclusion linking it to Schrödinger's zitterbewegung and proposing a zitterbewegung interpretation of quantum mechanics. Research in this direction is still active. The second line of research was dedicated to extending geometric algebra to a self-contained geometric calculus for use in theoretical physics. Its culmination is the book Clifford Algebra to Geometric Calculus which follows an approach to differential geometry that uses the shape tensor (second fundamental form). Innovations in the book include the concepts of vector manifold, differential outermorphism, vector derivative that enables coordinate-free calculus on manifolds, and an extension of the Cauchy integral theorem to higher dimensions. Hestenes emphasizes the important role of the mathematician Hermann Grassmann for the development of geometric algebra, with William Kingdon Clifford building on Grassmann's work. Hestenes is adamant about calling this mathematical approach “geometric algebra” and its extension “geometric calculus,” rather than referring to it as “Clifford algebra”. He emphasizes the universality of this approach, the foundations of which were laid by both Grassmann and Clifford. He points out that contributions were made by many individuals, and Clifford himself used the term “geometric algebra” which reflects the fact that this approach can be understood as a mathematical