The dispute between empiricism and rationalism begins within epistemology, the study of knowledge. Epistemology attempts to answer the questions: what is knowledge?, what can we know?, and what is the difference between opinion an knowledge? The study of knowledge began in Greece with the Pre-Socratic thinkers, as far back as the sixth and fifth centuries B.C.E. Zeno, a Pre-Socratic, is the first thinker to bring about the two schools of philosophy, rationalism and empiricism, which would grow to become a popular focus among other philosophers. Rationalism is defined as the epistemological view that true knowledge is derived from reason and from within the mind. This school of thought is based off of the a priori: truths that can be known independently of observations, and innate ideas: ideas believed to be present from birth. Empiricism, on the other hand, is the view that true knowledge is derived from sense experience. Empiricists believed that a priori and innate ideas were none existent, and rather all significant knowledge came from the a posteriori, the belief that truth is established only through observation. Zeno chose to focus on information derived from mathematics or rationalism, because he believed this information to be certain. He thought that information derived from the senses, or empiricism, could be deceiving. From there, the way by which we obtain knowledge continued to be argued over. During the seventeenth and eighteenth centuries, philosophers began to take sides as to what they believed was the source of knowledge. They formed two groups: the Continental rationalists and the British empiricists.