" Take, for instance, your favourite pair of wool socks. You get a hole in the big toe - and you darn it. Soon the darned holes are more of the sock than the original wool. Eventually the whole darned sock is made of different wool. Yet it's the same sock. In relation to its looks and in relation to its partner on your other foot, it is still the same sock. They go together and lie together in the drawer, and even in relation to itself, its identity, it is the same sock, though it is different.
Here philosophers can apply Plato's archetypal ideas of Sameness and Difference. The sock is entirely different from the original as far as the wool goes, but its shape has remained the same. It never becomes a different sock, despite the radical material alteration. Its material is different; its form is the same.
By "form", philosophers mean the look of the sock, for which you recognize it as a sock (tube socks raise conceptual problems!) When can a sock not look like a sock and still be a sock? Philosophers also mean by "form" the socks's function as a match to its partner and to your foot (form following function). A third meaning interests us most: form as the active principle governing the way the new wool integrates into the old sock. Form is thus visible shape, and the shaping force of the visible. Do you see that we are getting closer to the notion of character? "
James Hillman in The Force of Character and the Lasting Life