The issue is, if you word it like that, you are assuming what you are trying to show. If you have a brick, and you break it in two pieces, there's no way to "tell" gravity that the brick is now in two pieces, that much is true-- so it argues that different amounts of the same thing should fall the same. But if you further assume that the gravitational force depends only on how many grams you have, and not what it is made of (or the chemical processes going on inside, as alluded to by Nereid ), then you are assuming what you are trying to show. If grams are defined using the force of gravity, then you have a circular argument, and if grams are defined in terms of the measured inertia of the object, then you still have no way to know if different substances, rather than different amounts of the same substance, will fall the same. For example, why does a proton fall the same as a neutron? In other words, even though we expect both gravity and inertia to be additive properties of aggregate objects, you still can't break protons up into neutrons without changing them in a more fundamental way.

Having said that, I should point out that nevertheless your suggestion is a very good one for explaining why big bricks fall the same as small bricks, and that's really the main misconception people have, not the issue of why different substances fall the same.