Part of a page from John Duns Scotus's book Commentaria oxoniensia ad IV libros magistri Sententiarus, showing the words: "Pluralitas non est ponenda sine necessitate", i.e., "Plurality is not to be posited without necessity"

Manuscript illustration of William of Ockham

Possible explanations can become needlessly complex. It might be coherent, for instance, to add the involvement of leprechauns to any explanation, but Occam's razor would prevent such additions unless they were necessary.

Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660). Future positions of the sun, moon and other solar system bodies can be calculated using a geocentric model (the earth is at the centre) or using a heliocentric model (the sun is at the centre). Both work, but the geocentric model requires a much more complex system of calculations than the heliocentric model. This was pointed out in a preface to Copernicus's first edition of