If it is uncertain that external objects exist, how can we then have knowledge of them but by probability. There is no reason to doubt the existence of external objects simply because of sense data.
Russell guides the reader through his famous 1910 distinction between "knowledge by acquaintance and knowledge by description" and introduces important theories of Plato, Aristotle, Ren? Descartes, David Hume, John Locke, Immanuel Kant, Georg Hegel and others to lay the foundation for philosophical inquiry by general readers and scholars alike.