Comment by DrScientist

I always liked the Arthur Kornberg quote,

'don't waste clean thought on a dirty enzyme'

The specific problem he was facing was looking a DNA polymerases - and the problem with enzymes is they are catalytic - so a 1% impurity at the protein level might account for 100% of the activity you are measuring and wrongly ascribing to the 99% of purified protein.

So much of science is trying not to fool yourself.