Europe's 16th century was a time of great religious upheaval and disorder. Ardent opposition to modifying medieval customs resulted in the Thirty Years War, strict limitations for learning, and significant resistance to new ideas. Monumental discoveries, like those of Copernicus, Kepler and Galileo were swept under the table because they delivered a crushing blow to all geocentric views--the view preferred by the church. The curiosity of such men, however, must have been contagious--for emerging from the flames of the Reformation, was the individual: eager to learn and willing to doubt. The philosophy of the 17th century Frenchman René Descartes, which culminated with his momentous cogito, ergo sum, embodied the pervasive intellectual transformation of the era. For Descartes, everything was open to doubt except conscious experience, meaning the common denominator for all of our knowledge is ourselves. Re-emphasizing the importance of the individual hearkened back to Socrates who claimed we should know ourselves before tackling loftier metaphysical issues--and this is exactly the turn philosophy then took, which is why Descartes is often considered the "father of modern philosophy."
In trying to better understand the self, it seemed natural for Descartes and succeeding philosophers to study the nature of knowledge in attempt to understand how it is we know what we know. This branch of philosophy is called epistemology and it replaced metaphysics and ontology as the most fervently pondered sect of philosophy during this time. The history of epistemology has since seen some of its most prominent characters divided into two categories: the rationalists and empiricists.
In accepting our existence as our founding axiom, Descartes argued that we are able to then syllogistically build more advanced truths by deducing from already established truths--that all possible knowledge of the world is inherently available, regardless of our relationship with the world itself. This is essentially what rationalists believe. Rationalists believe that knowledge is inborn and is simply waiting for us to seek it out. Descartes, along with Spinoza and Leibniz are typically considered the "Continental Rationalists." Plato's universals and particulars are an example of classical rationalism. For Plato we are able to conceive the perfected version of something, a triangle for instance, despite never encountering one in the external world--while our senses may fool us into believing we are experiencing a perfect triangle, there is likely to always be an out of place molecule or interference making it imperfect. The reason we are able to conceptualize a perfect triangle without ever experiencing one with our senses is that we have mentally tapped into the realm of "universals", which holds the perfect form of all things. 'Particulars' are the mere imperfect adaptations of universals which we experience in the material world. Plato explains these ideas beautifully in his "allegory of the cave" in The Republic.
It follows then that empiricism, the seemingly diametric opposite of rationalism, holds that it is sensory data rather than inborn mental faculties that create knowledge. The spirit of the doctrine is expressed in Aristotle's claim: nihil in intellectu quod prius non fuerit in sensu: there's nothing in the intellect that wasn't first in the senses. Where Plato was a romantic, Aristotle was a scientist; he felt that it is imperative that we trust our senses, for what else have we to trust? It was this sort of ideology that penetrates both Aristotle's philosophy and empiricism. It was the English philosopher John Locke in the 17th century, however, that spearheaded the empiricist movement in modern philosophy. There is a significant correlation between both Aristotle and Locke's exposure to science and their empirical views. Aristotle, unlike Plato, was extremely involved with science and studying things of the world; Locke was living in the crest of the Scientific Revolution. Locke's An Essay Concerning Human Understanding was a blatant rejection of Descartes rationalist views. It was in this essay that Locke declared that in its natural state, the mind is a tabula rasa, or a blank slate. This is the fundamental empirical view, that all of our knowledge is derived exclusively from experience, or a posteriori; although there it is much debate between empiricists as to how exactly our perception of an object is transformed into knowledge.
Other notable empiricists following Locke were Bishop George Berkeley and David Hume. Berkeley was a subjective idealist, stating we can only know our own mind and ideas. He believed that objects exist only insofar as someone is able to perceive them; and that God was the eternal perceiver--therefore assuring us that objects still exist when humans are not perceiving them, and that trees do in fact make noise when no one is around to hear them fall.
David Hume, often dubbed "the ultimate skeptic" argued that since we can only know things inductively, we are unable to detect the effect in the cause. While we have habits, for Hume, we aren't actually able to know anything. Locke, Berkeley and Hume became known as the "British Empiricists."
