How can we know what is real and what is imagined? Here we have a ruler. But what is a ruler? How can we define a ruler? By what it's made of, by its purpose, by any of its other qualities?
Most of us would agree that a ruler is straight. Right? But look what happens when I put the ruler into the water. It no longer appears straight, but bent. If our senses can deceive us about this then is it also possible that when you look at your self in a mirror, you might not be seeing your real self?
What is the Self? Is the Self your legs, feet, fingers, toes or face? Many people who have lost their legs complained of feeling pain in the leg which had been amputated , so, are our senses playing tricks on us? Are we dreaming our existence? This was the question René Descartes asked himself.
René Descartes was a seventeenth century French Meta physicist. He is regarded as the first modern philosopher because of his work on the relationship between mind and body. In order to answer his own question, his starting point was to doubt the existence of everything. However, he concluded that if 'my'' senses can deceive 'me', there must be a 'me'' that can be deceived into sensing pain in an amputated leg. This 'I'' can doubt and, for Descartes, doubting is a proof of thinking and this in turn is a proof of existence "I think, therefore I am".
If nothing else is known, at least 'I'' know 'I'' am a thinking substance, but for everything else, doubt remains.