This book highlights foundational issues in theoretical physics in an informal, open style of lecture. It expresses the flow of ideas in physics -- from the period of Galileo and Newton to the contemporary ideas of the quantum and relativity theories, astrophysics and cosmology -- as explanations for the laws of matter. Rather than presenting the ideas of physics as a fait accompli, the book leaves it up to the reader to decide which of these 20th-century ideas in science will carry over to the 21st century for our further comprehension of the laws of nature in all domains, from that of elementary particles to cosmology.It is the contention of the author that our future progress in physics comprehension will only take place when the foundational controversies between the quantum and relativity theories are recognized and discussion is given to their resolution. The book, therefore, presents an attitude not normally taken in other present-day books on subjects in contemporary theoretical physics and cosmology.