What is reality really like? A current running through much of the philosophical thinking around the time of Socrates and Plato was that there is a difference between how the world appears and how it is. Our senses reveal one layer of reality but it is our minds that penetrate deeper. The world of appearances is a world in flux but underneath there must be a stable reality. For there is much that is unchanging. We recognise kinds of things – badgers, daffodils, mountains – and whilst members of these kinds are born, change and die, and differ from one another in ever so many ways, the kind-defining essence doesn't change. We see here the key rationalist idea that knowledge is a priori knowledge of necessary truths
Plato said that kinds were defined by the transcendental forms. He presented a number of arguments for the existence of these things. Prior to our incarnation, our souls existed in the realm of forms where we learned about these essences. In our terrestrial state, we cannot recall what we know. Socrates considered himself a "midwife to knowledge" instead of a teacher, helping his interlocutors to draw out what they don't know that they know. The example of Meno and the slave-boy shows this idea clearly.
Like many philosophers, Plato was also fascinated by mathematics. We are able to tap into a universe of truths that are non-sensible: we do not see numbers and we do not see the perfect geometric forms. Once again, we see the difference between the powers of the mind and the powers of the senses.
It was in the 17th century that the debate between the rationalists and the empiricists came to a head. Philosophers such as Descartes and Leibniz emphasised the power of reason over the senses. Descartes argued that our senses were fallible and that we could not rule out the possibility of the demon deception hypothesis on the basis of sensory evidence alone. Descartes argued that he knew he existed, as a mind, on the basis of reflection alone: when I think, I cannot fail to be aware of myself as existing as that thinker (cogito, ergo sum). Having proved that he exists, Descartes argued that God exists. Since God is no deceiver, he would not have given us senses that systematically mislead. But let us not overemphasise the powers of the senses. Descartes argued that even with material things, it is reason that exposes their essences. In his piece of wax reasoning, he argued that the senses merely reveal a succession of impressions: it is reason that grasps the underlying and enduring substance as extended (and filled space).
Plato and Descartes believed that we are born with concepts and knowledge. In Descartes' case, there was a religious motive: we are all born in the image of God. We discover more about the world primarily through metaphysical reflection. The philosopher Francis Bacon, an early empiricist, famously dismissed this rationalist approach to knowledge. He compared rationalists to spiders who spin "complex metaphysical systems out of their entrails". Empiricists get their hands dirty: like bees gathering pollen, they gather knowledge about the world and only then reflect on it. Around the same time as Bacon, many new discoveries were being made that shook the prevailing views of reality. The Earth was dethroned from its position at the centre of the universe by Copernicus. A new star (a supernova) was observed by Tycho Brahe in 1572 – yet the heavens were supposed to be timeless and unchanging. Galileo discovered the moons of Jupiter – again, everything clearly didn't revolve around the Earth. Later in the 17th century, scientist-philosophers such as Newton, Boyle, Gassendi and Huygens would revolutionise our understanding of reality.
The original empiricist manifesto was written by John Locke. In his Essay Concerning Human Understanding, he sought to show how a mind that was blank at birth – a tabula rasa or blank slate – could come to be...