Now, whenever I conceive of any material or corporeal substance, I am necessarily constrained to conceive of that substance as [a] bounded and [b] as possessing this or that shape, [c] as large or small in relationship to some other body, [d] as in this or that place during this or that time, [e] as in motion or at rest, [f] as in contact or not in contact with some other body, [g] as being one, many, or few--and by no stretch of imagination can I conceive of any corporeal body apart from these conditions.What struck me at first as dubious is condition [c]--Galileo can only conceive of some material body in relation to some other material body. This means that Galileo couldn't conceive of the existence of, say, a single bar of gold. For any body he can conceive it must be conceived in relation to some other body. So he can only conceive of two bodies or more.
But perhaps the idea is that in order to conceive of a single bar of gold one must conceive of a left half and a right half of which the bar is composed--each of which the bar is larger than.
But what about a point particle, such as an electron which has no extension? Conceiving it would violate [c] as well as [a] and [b]. But electrons can be conceived.
Perhaps Galileo is confusing imagining or picturing with conceiving. One can't picture an electron though one can conceive of an electron.
Alternatively--though I am sure this was not Galileo's view--a metaphysician of an empiricist bent could hold that the above criteria do more or less accurately provide the criteria for conceiving of a material body while being an instrumentalist rather than a realist about "material substances" such as electrons.