Philosophical realism, or simply realism, is a philosophy that begins where we all begin: with our common sense, our common everyday experience. When Samuel Johnson famously dismissed Berkeley’s idealist philosophy with his “I refute Berkeley thus” and then kicked a rock, he was asserting realism in contrast with idealism.
There are basically two kinds of anti-realist philosophy: idealism (or ideology) and materialism (or naturalism). Idealism begins with an idea that is asserted to be the principle of reality. Materialism begins with physical matter and everything is asserted to be reducible to this matter (or nature).
Realism as a philosophy began with Socrates, Plato, and Aristotle, the three leading lights of classical thought. Plato’s realism of Forms was rather extreme but his student Aristotle taught a moderate realism with a duality of form and matter. Plato’s realism motivated Neo-Platonism in later ancient and early medieval times. When Aristotle was rediscovered, his realism motivated Scholasticism in the Middle Ages.
The Scholastics were limited in their understanding of Aristotle and ended up giving him a bad name so that the early moderns opposed all things associated with Aristotle. Ironically, people such as Francis Bacon who assailed Aristotle also incorporated key elements of Aristotle in their own philosophy.
Modern realists include the American philosophers C. S. Peirce, W. V. O. Quine, M. J. Adler, and Thomas Nagel. While they mostly lack a religious faith, they are not inimical to true religion. Their realism puts them at odds with many of their contemporaries and closer to true religion than might appear at first.
Realism is an open philosophy, contrary to the anti-realist philosophies which have decided what reality is from the start. Realism is open to reality, however that may turn out to be. Realism is consistent with the common sense that people in general have and so is a way of engaging people in a common pursuit of the true, the good, and the beautiful.