This book is intended for science and engineering majors who are required to take calculus and are looking for a more intuitive way of understanding it. This is a non-rigorous infinitesimal approach which focuses on differentials of variables that represent physical quantities rather than derivatives as limits of of mathematical functions. In science variables are related in equations so this is the focus rather than on dependent and independent variables of functions. These methods were originally conceived by G. Leibniz over 300 years ago and have been used successfully by scientists ever since.