A rock is not physically a material object, it is a region of space where the electrons, protons and neutrons are differently arranged, and that region is fuzzy, difficult to determine; but as physical beings, as monkeys, we recognise its oneness, that's necessary for our survival in this physical world, we see this blurred outline of a rock, we feel it's weight in our hand, we observe its practical difference from two rocks. Just as we recognise twoness in a pair of rocks, fish, apples, threeness in a triple of parrots, of carrots, we abstract those out into 1, 2, 3, ...