There is a cave where people are tied in chains. Their heads are fixed, staring at the back wall, where images appear. There is a fire behind them and in front of the fire pass a series of objects which cast shadows on the wall of the cave. The inhabitants of the cave have lived here all their lives and to them the shadows are all they know of reality. Indeed, were someone to take one of these people and release them from their chains, how would they even comprehend the true nature of what they saw when they were shown the fire, and the shapes that cast the shadows?
The cave is a metaphor that was used by Plato to describe our understanding of the world. For Plato the real world was made up of Forms, ideal versions of objects. He said that philosophers could break away from the chains and, after being disoriented by the truth, understand the way things really are. Once understanding was attained it was incumbent on the philosopher to bring his wisdom back to the people who were still convinced that the shadows were the real world. Aristotle considered the pursuit of philosophical knowledge to be the ultimate happiness. (Spinoza considered it highly ethical too, though perhaps it is not surprising that many philosophers consider their work to be of high moral/social/ethical value…) So how do we break the chains and join Plato in delivering good council to the world, and join Aristotle in his happiness?
While Plato’s Forms have fallen out of favour his metaphor of the cave still applies. We do not see the real world, we live in the virtual reality that is our minds. Our vision is put together not just from photons hitting light receptors in our eyes, but by complex algorithms in our brains that recognise faces, and fill in the blanks in our blind spots. But more than this there are places that our regular senses cannot probe. By my eyes alone I cannot determine the chemical composition of the Sun, or see a virus. Atoms are made of a dense centre and a strange cloud of probabilities and electric charge, all of which are hidden to me, instead I see the shadows on the cave wall.
Fortunately we have some ways of breaking the chains. The development of glass lenses led to the first telescopes being used in the 16th century. By using the properties of the lens to magnify his vision Galileo was able to see the phases of Venus and so prove the Heliocentric model of our solar system. He was also able to see four moons orbiting Jupiter, other worlds that no human before could ever have known.
The glass lens also led to the microscope and visions of tiny creatures invisible to the naked eye. Robert Hooke wrote a best seller published in 1665, which contained his drawings of objects seen through his microscope, including a louse, a fly’s eye, and plant fibres which contained what he called ‘cells’. The Dutch scientist Leeuwenhoek working around the same time was the first to discover micro-organisms and single celled creatures.
In 1800, during some work testing filters to observe sunspots, the astronomer William Herschel placed his thermometer beyond the red end of a rainbow produced by a prism. He was trying to establish the ambient temperature of the room but noticed that even though he could see no light, there was extra heat produced outside of the rainbow. He had discovered infrared light, unseen to human eyes, but detectable by human instruments.
In 1801, Johann Ritter discovered that the light at the other end of the spectrum, again beyond our ability to see, was especially good at darkening silver salts. This was the first detection of ultraviolet light. Within a hundred years the spectrum from X-rays to Radio waves was being studied and as astounding and unsettling as these ideas were, they were brought back to the cave and incorporated into our idea of how the world works. We cannot see them, but we see their effects.
Our instruments can guide us out of the cave, but so too can the stories and results of others who have looked at the fire itself. If you have never looked through a telescope at Saturn, or seen the moons of Jupiter then I would recommend it. But the instruments to see even further can be expensive or rare and not everyone can directly experience the new visions that scientists find every day. Nor could we lay people instantly understand the results of say, the Large Hadron Collider just by looking at the computers in Geneva. In these cases we wait for the natural philosophers to return from looking at reality, confusing at that reality may be, and try to understand through metaphor what we can never see ourselves.