Although it is difficult to say exactly what Thales accomplished, if you take what he's given credit for it is immense. By your criteria he would qualify as an answer.
Although he lacked the rigor of proof, he was the first mathematician we attribute a mathematical result to - namely Thales' Theorem used to get the distances of ships at sea.
He was also the first philosopher. With his contention that everything is water we have the first theory that says everything is one.
Also with his contention that everything is water we have a testable claim. This makes him the first scientist.
He also was an astronomer, predicting the eclipse of May 28th, 585 BC.
He even dabbled in business, by legend. There is a famous story attributed to Aristotle, and others, about him. If true, Thales predicted the weather and foresaw a good olive harvest for the coming year. He then bought all the olive presses at a discount, and rented them out during the harvest. This would be the first recorded use of futures to turn a profit.