It was a thought experiment. He imagined an anvil and a hammer.
According to the then accepted theory, the anvil would fall faster
when they were separate. If you then tied them together with stout
twine and dropped them, the anvil would pull down on the hammer
making it fall faster, but the hammer would pull up on the anvil
making it fall slower. This leads to a logical inconsistency which
showed him that the original premise was faulty. He thus concluded
that objects of any mass would fall at the same rate.