Meditations on First Philosophy (1641) by René Descartes: The search for the foundations of knowledge, famously employing a method of systematic doubt. Descartes wrote in the Preface, "I would not urge anyone to read this book except those who are able and willing to meditate seriously along with me." The book is not a static exposition of finished doctrines, but a set of mental exercises that each individual must follow for himself. In so doing, Descartes maintained, each of us can become indubitably convinced first of his own existence, then of the existence of God, and finally of the essence of material things and the true nature of the human mind.

An Enquiry Concerning Human Understanding (1748) by David Hume: Hume argues that all knowledge originates from experience, either through sensory impressions or through reflections on those impressions. He criticizes the notion of innate ideas and challenges traditional metaphysics, arguing that concepts like causation and substance are not based on reason but rather on habitual associations of ideas.

Prolegomena to Any Future Metaphysics (1783) by Immanuel Kant: Kant attempts to reconcile rationalism (à la Descartes) and empiricism (à la Hume). He argues that our minds actively structure experience through a priori concepts like space, time, and causality, allowing us to understand the world. He outlines the limits of human knowledge, emphasizing that we cannot know things-in-themselves, only how they appear to us.

Essay on the Freedom of the Will (1839) by Arthur Schopenhauer: The winning entry in a competition held by the Royal Norwegian Society of Sciences, Schopenhauer argues that the will is not free but rather determined by a force he calls the "Will to Life." He explores the nature of human motivation, arguing that our actions are not freely chosen but rather driven by unconscious desires and instincts.