Full Idea
No one is so wise regarding the natural world as to know with certainty all the truths that concern the nature and properties of a single fly, or to know the proper causes of its color and why it has so many feet, neither more nor less.
Gist of Idea
No one even knows the nature and properties of a fly - why it has that colour, or so many feet
Source
Roger Bacon (Opus Maius (major works) [1254], I.10), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 23.6
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.543
A Reaction
Pasnau quotes this in the context of 'occult' qualities. It is scientific essentialism, because Bacon clearly takes it that the explanation of these things would be found within the essence of the fly, if we could only get at it.