Plato (424/423 BC – 348/347 BC), was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the foundations of Western philosophy and science.
“The essence of knowledge is self-knowledge,” said Plato. Since Socrates never wrote, and since our knowledge of his teaching comes through the pen of others such as Plato, it is difficult to draw the line between what the one said and the other. Be that as it may, in his works, Plato repeatedly reiterates his teacher’s call to self-knowledge:

I must first know myself, as the Delphian inscription says; to be curious about that which is not my concern, while I am still in ignorance of my own self would be ridiculous.
I may be a simpleton, but in my opinion, only that knowledge which is of being and of the unseen, can make the soul look upwards.

Inexpressible Self-knowledge

“I feared to see myself at last altogether nothing but words,” confessed Plato in one of his works. Words, after all, are only labels that point to another reality. Naked truth must therefore be wordless, and the philosopher in search of truth must watch lest he bury the treasure he seeks under the debris of words.
At times, Plato’s inquiries into the Self echo the Hindu Upanishads, wherein the Self is contrasted by the body. The path to self-knowledge, then, is described as identical to the effort of dissecting body from soul:

If man’s attention is centered on appetite … all his thoughts are bound to be mortal and he can hardly fail to become entirely mortal.

Philosophy as the means to Self-knowledge

The Self exists as a potential to be realized. Plato expresses this poetically: “The soul is a helpless prisoner, chained hand and foot in the body, compelled to view reality not directly, but only through its prison bars, and wallowing in utter ignorance,” he says. “Philosophy takes over the soul in this condition and encourages it to collect and concentrate itself by itself, trusting nothing but its own independent judgment.” Philosophy is, therefore, the art and science of dissecting soul from body. It is a task available to all yet pursued by few:

In every man there is an eye of the soul which is more precious than ten thousand bodily eyes.

Finally, as an encouraging teacher, Plato promises a reward for those who persist in attaining self-knowledge. Although difficult, the end justifies the means and the darkness of self-ignorance can be dispelled by the intelligent guidance of philosophy:

At last, in a flash, understanding blazes up, and the mind, as it exerts all its powers to the limit of human capacity, is flooded with light.

Further Reading:
Upanishads on Know Thyself
Socrates on Know Thyself
George Gurdjieff on Know Thyself