Albertus Magnus Albertus Magnus inspecting a stone beside an open book

What makes a stone the kind of stone it is?

Albertus Magnus (c. 1200–1280), a German Dominican friar and natural philosopher, opens his investigation of gemstones with a problem of explanation. Saying that a stone possesses a particular power gives a name to an observation or a report. What is it about this particular stone that would make it behave in this way? In the second book of his work on minerals Albert weighs several answers before offering his own.

What he means by form

Having rejected all these views, we say with Constantine and certain others that the power of a stone arises from its very kind and substantial form.

– Albertus Magnus, De mineralibus, 1519, unpaginated.[1]

Hearing the word “form,” someone holding a gem might think of the outline that a finger can trace. Albert’s example points elsewhere. He asks his reader to consider the human capacity for thought. Taking the elements of a body separately would scarcely explain how a human being is able to think. In the case of the stone he likewise seeks something that makes its material a whole of a particular kind. His term for this is “substantial form.” The question of whether a specimen is round or angular would take the reader only a little way towards his meaning.

Two specimens of the same kind

A familiar difficulty emerges as the discussion proceeds. Even specimens assigned to the same kind can differ. For Albert the particular material helps determine how fully a capacity can act in it. His account can therefore recognise both a kind held in common and differences between specimens. Towards the end he even distinguishes what a stone still looks like from what it can still do. Colour and outline might remain familiar to the eye longer than the property whose cause set the whole inquiry in motion.