Most of the earliest philosophers conceived only of material principles as
underlying all things. That of which all things consist, from which they
first come and into which on their destruction they are ultimately
resolved, of which the essence persists although modified by its
affections—this, they say, is an element and principle of existing things.
Hence they believe that nothing is either generated or destroyed, since
this kind of primary entity always persists. […]
for there is some one entity (or more than one) which always persists and
from which all other things are generated.
All are not agreed, however,  as to the number and character of these
Thales, the founder of this school of philosophy, says the permanent
entity is water(which is why he also propounded that the earth floats on
Presumably he derived this assumption from seeing that the nutriment of
everything is moist, and that heat itself is generated from moisture and
depends upon it for its existence (and that from which a thing is
generated is always its first principle).
He derived his conviction, then, from this; and also from the fact that the
seeds of everything have a moist nature, whereas water is the first
principle of the nature of moist things