1637 – France
‘Cogito ergo sum‘ – The result of a thought experiment resolving to cast doubt on any and all of his beliefs, in order to discover which he was logically justified in holding.
Descartes argued that although all his experience could be the product of deception by an evil daemon, the demon could not deceive him if he did not exist.
His theory that all knowledge could be gathered in a single, complete science and his pursuit of a system of thought by which this could be achieved left him to speculate on the source and the truth of all existing knowledge. He rejected much of what was commonly accepted and only recognised facts that could intuitively be taken as being beyond any doubt.
His work ‘Meditations on First Philosophy’ (1641) is centered on his famous maxim. From this he would pursue all ‘certainties’ via a method of systematic, detailed mental analysis. This ultimately led to a detached, mechanistic interpretation of the natural world, reinforced in his metaphysical text ‘Principia Philosophiae‘ (1644) in which he attempted to explain the universe according to the single system of logical, mechanical laws he had earlier envisaged and which, although largely inaccurate, would have an important influence even after Newton. He envisaged the human body as subject to the same mechanical laws as all matter; distinguished only by the mind, which operated as a distinct, separate entity.
Through his belief in the logical certainty of mathematics and his reasoning that the subject could be applied to give a superior interpretation of the universe came his 1637 appendix to the ‘Discourse’, entitled ‘La Geometrie‘, Descartes sought to describe the application of mathematics to the plotting of a single point in space.
This led to the invention of ‘Cartesian Coordinates’ and allowed geometric expressions such as curves to be written for the first time as algebraic equations. He brought the symbolism of analytical geometry to his equations, thus going beyond what could be drawn. This bringing together of geometry and algebra was a significant breakthrough and could in theory predict the future course of any object in space given enough initial knowledge of its physical properties and movement.
Descartes showed that circular motion is in fact accelerated motion, and requires a cause, as opposed to uniform rectilinear motion in a straight line that has the property of inertia – and if there is to be any change in this motion a cause must be invoked.
By the 1660s, there were two rival theories about light. One, espoused by the French physicist Pierre Gassendi (1592-1655) held that it was a stream of tiny particles, traveling at unimaginably high-speed. The other, put forward by Descartes, suggested that instead of anything physically moving from one place to another the universe was filled with some material (dubbed ‘plenum’), which pressed against the eyes. This pressure, or ‘tendency of motion’, was supposed to produce the phenomenon of sight. Some action of a bright object, like the Sun, was supposed to push outwards. This push was transmitted instantaneously, and would be felt by the human eye looking at a bright object.
There were problems with these ideas. If light is a stream of tiny particles, what happens when two people stand face-to-face looking each other in the eye? And if sight is caused by the pressure of the plenum on the eye, then a person running at night should be able to see, because the runner’s motion would make the plenum press against their eyes.
Descartes original theory is only a small step to a theory involving pulses of pressure spreading out from a bright object, like the pulses of pressure that would travel through water if you slap the surface, and exactly equivalent to pressure waves which explain how sound travels outward from its source.