The importance of greek philosophy was not that its conclusions were always correct, but that for the first time it was based on:
force
astronomy
hard work
reason

The correct answer is REASON

The Greeks argued that our senses are faulty and cannot really be trusted. There is even an anecdote that a Greek teacher said there is no movement, and a student got up and started moving, and got scolded by the teacher: you need to argue by reasoning, not by showing things to our senses which can be fooled.

Only our reason cannot be fooled, they argued.