Berkley stated that because the senses were potentially faulty, everyone's sense perceptions and thus everyone's 'truth' was unique and variable. However, most empiricists like Locke believed that some (few) things could be known with certainty, like shape and color, even if other properties of things could not be known. The empiricists come from the Aristotelian rather than the Platonic tradition of philosophy, and had rigorous standards of truth based upon sensory experience rather than reason alone. Another way of phrasing the debate between empiricism and rationalism is that it is an essential conflict between the superiority of a posteriori reasoning vs. A priori reasoning.
A posteriori reasoning depends upon what we know about past events and information to make inferences, in short, observations and experience. A priori reasoning suggests just the opposite, suggesting that everything is there, if only we can learn to think correctly in a deductive manner. Thus geometry or deduction is the epitome of rationalism, in contrast to the scientific method deployed by empiricism. But eventually, empiricism's emphasis on science began to unravel, as David Hume's radical empiricism postulated that an individual could not create general laws about behavior, instead someone only make statements about specific instances. This truth-standard seemed to undercut the scientific value of empiricism, since even science requires some scientific, consistent laws to be valuable.
The conflict between empiricists and rationalists was to some extent resolved, or at least bridged in Immanuel Kant's Critique of Pure Reason. Kant, later called the founder of German Idealism, attempted through his categorical imperative to provide a moral law and guide for human behavior, suggesting that certain rules of conduct must be observed and obeyed as if they were to set the law for all time (reflecting the deductive logic exemplified in Platonism and rationalism). But he bridged both of the questions that arise in previous thinker's philosophies by suggesting that neither the rationalists nor the empiricists could be judged entirely correct. Kant stated there was a mind that existed before 'experience,' contrary to the empiricists' assertion. For 'experiences' to occur there must be a pre-existing mind. But unlike the rationalists, Kant did not believe that there were pre-existing ideas in the pre-experiential mind, like the existence of God.
Kierkegaard, along with Kant, is called one of the founders of modern philosophy, but Kierkegaard is particularly associated with existentialism, the radical freedom and sense of loneliness and alienation human beings experience in the world. Man exists in a state of anxiety, Kierkegaard believed, which could temporarily be remedied by pleasure, but a truly ethical man must make a leap of faith beyond rationality and believe in God, beyond all reason, to feel fulfilled. However, not all of the intellectual descendents of Kierkegaard would find such comfort in faith, although this devaluing of rationality and rationalism would continue to flourish. For example, Nietzsche proclaimed the 'death of God.' Nietzsche also suggested that truth, and purpose in meaning in life was far from knowable, either by logic or established truths. Nietzsche stressed a pagan philosophy of freedom from socially imposed conventions like religion. Religious and social institutions had nothing to do with truth, Nietzsche believed, but were made by men and thus could be done away with by men.
Rationalism did remain, to some extent, as the 19th century saw the birth of the social sciences, and a new form philosophy that of explaining and addressing social ills, as exemplified in Auguste Comte, Karl Marx, and William James' pragmatism. Comte attempted to rationally quantify and classify society, as did Marx. But Marx stressed the need to address the cause of all human conflict (class relations) and by eliminating class, an ideal society was possible. Marx's philosophy was focused on the real world, historical observation, but also postulated an ideal society, drawing upon both rationalist and idealist conceptions of the human condition. James' scientific pragmatism suggested a 'what works approach,' and treated the conflict between empiricism and rationalism in terms of the psychological orientation of their different advocates. James did not believe in 'truths' that existed outside of the psyche that could not be verified. Human beings need to deal with real-life experiences, although he also believed that psychologically humans also need an idealistic sense that what they are doing has a larger purpose.
The psychological quandaries created by the aftermath of the world wars, however, created an even greater spiritual crisis for philosophy. Drawing from Nietzsche's strains of freedom and atheism, Jean-Paul Sartre's existentialism emphasized human freedom in a world with no inherent, fixed truths, much as the Sophists did so long ago. Human anxiety was the result of this freedom, for it was humanity's choice to learn how to cope with such responsibilities, and to create new ways of being in a world that was a moral void. Existentialism's stress upon the subjective nature of truth is reflected in the most dominant strains of philosophy today, that of postmodernism, which stresses the constructed nature of truth in a world that is neither immoral nor moral, but amoral.
Stumpf, Samuel Enoch & James Fieser Socrates to Sartre and Beyond: A History of Philosophy. 7th Edition. New York: McGraw-Hill Higher Education, 2003.
Samuel Enoch Stumpf & James Fieser, Socrates to Sartre and Beyond: A History of Philosophy, 7th edition, (New York: McGraw-Hill Higher Education, 2003), p.80.