Berkeley (1685–1753) argues that claims about mind-independent objects lead to skepticism about knowledge. More specifically, he is concerned with the idea that a mind-independent object is one that cannot be known, that there is something left over or beyond the idea itself. Berkeley solves this problem by arguing that objects are nothing more than collections of sensations, and, as ideas are equivalent to sensations, objects are nothing other than ideas. An apple, for example, is nothing other than its particular color, texture, weight, taste, smell, and so forth.
Descartes (1596–1650) argues that reason alone provides a certain foundation for knowledge about the world. Through systematic doubt, Descartes arrives at the certainty of his existence as a thinking thing, the existence of God, and, eventually, the existence of an external world.
In An Enquiry Concerning Human Understanding, Hume mounts a sustained argument from a purely empirical starting point. All knowledge is derived entirely from sense impressions, both internal and external. Any idea, therefore, that cannot be traced to an experiential source is, strictly speaking, illegitimate. Consequently, ideas such as a soul or persisting self, freedom, and God should be discarded.
Hume's argument against knowledge of causation had a profound effect on Kant's thought. It was Hume's skeptical conclusions about the idea of causation that inspired Kant's masterwork, the Critique of Pure Reason. Kant (1724–1804) effectively synthesized the two strains of philosophy: rationalism and empiricism. He argued that certain a priori concepts, such as causation, are required for experience. In addition, he argued that concepts require raw material on which to work up ideas: "thoughts without content are empty; intuitions without concepts are blind."
In his major epistemological work, An Essay Concerning Human Understanding, Locke (1632–1704) argues for, among other things, the claim that there are no innate ideas, that is, that the human mind is empty at birth and is furnished entirely by sense experiences.