 Scepticism and Animal Faith (1923) introduces Santayana’s mature naturalism. In summary, he maintains that knowledge and belief are not the result of reasoning. They are inescapable beliefs essential for action. Epistemological foundationalism is a futile approach to knowledge. A more promising approach is to discern the underlying belief structures assumed in animal action and imposed by natural circumstances. The foundations for this approach are rooted in Aristotle’s concept of activity and the pragmatic approach to action and knowledge. Explanations of natural events are the proper purview of scientists, while explications of the meaning and value of action may be the proper sphere of historians and philosophers. Even so, both scientific explanations and philosophical explications are based in the natural world. Meaning and value are generated by the interaction of our physical makeup, which Santayana calls “psyche,” and our material environment.