1. Socrates ↑
The mind is still one of the biggest mysteries in science. It appears to be so different to other physical substances that throughout most of history, people have believed that it's not a physical substance at all.
The first Ancient Greek philosophers held a rationalist approach to the theory of knowledge. This means that they believed true knowledge comes from the mind, which is rational, and not from the senses, which can be tricked.
Socrates was a mind-body dualist. This means he thought that the mind is composed of a different substance to the brain. The argument that the mind is made of a substance that does obey the laws of physics is known as physicalism or materialism.
Socrates believed that the mind has an irrational part, which is controlled by emotions, and this is drawn to the body. Once the mind and body merge, the mind is limited by what we are able to perceive with our senses. The rational part of our mind mostly remains beyond our conscious knowledge, however Socrates believed that it's the job of philosophers to connect to the rational mind in order to become a whole person. Once this is achieved, a rational person will see things for what they really are.
2. Plato ↑
Socrates' pupil, Ancient Greek philosopher Plato, elaborated upon this theory in The Republic, where he described how the things we perceive on Earth are really composed of ideas, or forms. A form is an eternal and perfect concept, something that is strived for but never actualised on Earth.
All horses, for example, are united by the concept of 'horse', an ideal that all horses on Earth were built to resemble. But it is not just objects and individuals that have forms. Forms also apply to abstract concepts such as beauty.
Plato claimed that all of the forms exist outside of the realm of regular perception, in the 'realm of the forms'.
Plato did not trust sensory information because we can confuse reality with the imagination. The most extreme cases happen when we dream or hallucinate but this also occurs when we confuse one object for another.
Plato showed that we are often presented with illusions of this kind. A stick, for example, can appear bent in water, yet when we pick it up, we will find that it's straight. Things are not always what they seem, and we are not always aware that we are making these mistakes.
Plato's allegory of the cave describes how we are analogous to people who spend their lives in caves, only looking at shadows. These people will come to believe that only shadows exist, and when someone tells them of the world of light above, they do not believe them.
Plato claimed that it takes a philosopher to leave the cave, and see the world as it truly is. Like Socrates, Plato believed this could only be achieved through rational introspection.
As with Socrates, Plato's belief that the mind is separate from the body came from the need to explain human intellect, animals did not seem to possess anything similar and it couldn't be explained mechanically.
The forms explain how the mind interprets the continuous stream of sensory data it's exposed to by recognising certain eternal concepts. If our intellect is composed of forms, then it is eternal and distinct from the body.
Plato did not believe that the mind exists in time or space and thought that it would return to the realm of the forms upon death.
3. Aristotle ↑
Plato set up an academy for philosophy, and one of its pupils was Ancient Greek philosopher Aristotle, who was born in 384 BCE. Aristotle rejected many of Plato's theories including rationalism.
In On the Soul, Aristotle argued that the mind doesn't have innate ideas, and compared it to an unscribed tablet, a 'tabula rasa'. Aristotle also rejected Plato's realm of the forms, arguing that the forms are concepts devised by people to categorise things.
Aristotle argued that the mind is a part of the human body, and so also rejected mind-body dualism. He did however, believe that intellect is different from any other part of the body. This is because our conscious range does not appear to be restricted in the way that our physical senses are.
Aristotle claimed that intellect does not have a corresponding bodily organ. This means that it does not exist in space, despite having a physical origin.