Thales was born in 624 BC in Miletus, a small town located on the west coast of Asia Minor. This city is a trade center. At first, he worked as a trader, so he often traveled abroad. In fact, he had traded to Egypt and Babylon (during the reign of Nebuchadnezzar). Meanwhile, he uses his free time to study astronomy and geometry. He is interested in using these tools to predict solar eclipse events every year. In fact, he also put forward a proposition known as the Thales Theorem, the contents of which are as follows:

- The circle is divided by 2 lines through its center, called diameter
- The magnitude of the corners of the base of the isosceles are equal
- Vertical angles that are formed from 2 parallel lines cut by a cross line will be the same
- If a pair of sides, a pair of angles located on that side, and a pair of angles that lie opposite that side are the same size, then the two triangles are said to be the same and congruent
- Triangles whose base is known and have a certain angle, can be used to measure the distance of the ship.

Thanks to the theorem, he was known as a pioneer of Greek mathematics and philosophy. Evidence of his achievements has been traced through the records of his students such as Aristotle and Eudemus of Rhodes (circa 320 BC). Eudemus notes that Thales was a person who changed geometry into formal forms, so that it could be learned by everyone, because he based himself on principles while investigating theorems with intellectual perspectives. Thales spoke of lines, circles, and forms others by imagining (abstract). According to him, the line is not just something that can be streaked and seen on sand, but is an object that is trapped in the imagination. In other words, straight lines or round circles are in our mentality.

Thales is known as an applied mathematician. As for some notes of his greatness, among others, he could measure the height of the pyramid by measuring the height of the shadow using a stick, predicting the occurrence of a solar eclipse, determining the time of a year consisting of 360 days (this has long been known by the Egyptians), and calculating the distance of ships in the ocean by means of proportions or matching triangles.

He also stated that a solar eclipse would occur on May 28 or September 30 in 609 BC. And, solar eclipses occur every period of 18 years 11 days. The accuracy of this prediction made him famous, even he was immortalized into 1 of 7 sages (sage) found in Greek saga.

He had many students, including Anaximander, Anaximenes, Mamercus and Mandryatus. The most famous student is Anaximander. Furthermore, Anaximander succeeded in replacing Thales at Miletus.

Thales is also touted as the first mathematician to formulate theorems or propositions. This tradition became clearer after being elaborated by Euclid. Thales, who had laid the foundation of mathematics as an applied science before the appearance of Pythagoras, died in 550 BC.