No known writing survives from Thales. It's been said he did not write down his teachings. What we know of him comes from his students and those who came after him. In addition to being a philosopher, Thales was also known as a mathematician, astronomer, and engineer.
Things He Said
“The oldest of beings is god; for he is unborn.”
“The most beautiful thing is the world; for it was made by god.”
“The biggest thing is place; for it contains everything.”
“The fastest thing is mind; for it races through everything.”
“The strongest thing is necessity; for it rules over everything.”
“The wisest thing is time; for it discovers everything."
He said that death is not at all different from life.
Someone said, “Then why don’t you die?”
He answered, “Because there is no difference.”
To the man who wanted to know which came about earlier, night or day, he replied, “Night, earlier by a day.”
Someone asked him whether a man escapes the notice of the gods if he commits injustice; he answered, “not even if he intends to.”
And to the adulterer who asked whether he should swear that he had not committed adultery, he answered, “Is not perjury worse than adultery?”
When asked what is difficult, he answered, “to know oneself”;
What is easy, “to give advice to someone else”;
What is most pleasant, “to have success”;
What is divine, “that which has neither beginning nor end”;
What is most unheard of , he said, “an old tyrant.”
How one could most easily endure misfortune? “If one sees one’s enemies doing worse.”
How we could live best and most justly? “If we do not do ourselves what we blame others for doing.”
Who is happy? “He who is healthy in body, resourceful in spirit, well trained in nature.”
He says that we should remember our friends, be they present or absent
He says not to beautify our appearance, but to be beautiful in what we do.
He says, “Do not enrich yourself dishonestly, nor let any utterance set you against those who share your trust.”
He says, “The very same favours that you did for your parents, expect them from your children too.”
"Many words do not manifest a sensible opinion.
Search for one thing: what is wise.
Choose one thing: what is good.
For you will undo the endlessly talking tongues of chattering men."
Water is the principal element. In the beginning and in the end, there is water. It is from water that all things are formed. Earth is a globe that rests in water, it floats like wood and moves like a boat [space is fluid].
The stars are made of earth but are on fire.
There is only one world.
All things are full of gods.
Souls are things that move. Inanimate objects can have a soul if they provoke movement, such as magnets. Amber also has a soul.
Just 'cause I don't doesn't mean I can't.
People began insulting Thales due to his poverty, saying his philosophy must be useless for him to be so lacking in wealth. So one winter, using his knowledge of astronomy, he was able to predict when there would be a large crop of olives.
Using the little money he had, he paid deposits on ALL the olive-presses in Miletus and Chios, renting them cheaply since no one was competing with him.
As he predicted, there was a massive crop of olives, and when time came to harvest them, all at once many people needed the olive-presses he possessed. Thales rented the presses out at prices as high as he chose, and made a lot of money.
Thus demonstrating to all his naysayers that it was easy for him to become rich if he wish, but that wasn't what he wanted to do.
Aristotle, On the Heavens
Aristotle, On the Soul
Galen, Commentary on Hippocrates’ On the Nature of Man
Hippolytus, Refutation of All Heresies
Seneca, Natural Questions
Early Greek Philosophy, Volume II: Beginnings and Early Ionian Thinkers, Part 1. Edited and translated by André Laks, Glenn W. Most. Loeb Classical Library 525. Cambridge, MA: Harvard University Press, 2016.