We cannot observe external things without some degree of Thought; nor — William Whewell

"The Sight cannot decompose a compound colour into simple colours, or distinguish a compound from a simple colour"

"A Natural System is one which attempts to make all the divisions natural, the widest as well as the narrowest; and therefore applies no characters peremptorily."

"In Deductive Reasoning, we cannot have any truth in the conclusion which is not virtually contained in the premises."

"The antithesis of Sense & Ideas is the foundation of the Philosophy of Science. No knowledge can exist without the union, no philosophy without the separation, of these two elements."

"The solution is, that we do not see the image on the retina at all, we only see by means of it."

"Among the foundations of the Higher Mathematics is also the Idea of a Limit. The Idea of a Limit cannot be superseded by any other definitions or Hypotheses."