Opinion (from Latin opinor, to think) is a synonyme of belief, and measurably, too, of knowledge; but, while the last-named term can be applied to what is objectively and subjectively held as sufficient, and belief is applied to what is subjectively sufficient, opinion is properly applied only to a consciously insufficient judgment, or, as Sir Lewis has it: "The essential idea of opinion seems to be that it is a matter about which doubt can reasonably exist, as to which two persons can without absurdity think differently... Any proposition, the contrary of which can be maintained with probability, is matter of opinion" (Essay on Opinion). According to the last of these definitions, matter of opinion is opposed not to matter offact, but to matter of certainty. Thus the death of Charles I is fact — his authorship of Icon Basilike, an opinion. It is also used, however, to denote knowledge acquired by inference, as opposed to that acquired by perception. Thus that the moon gives light is matter of fact; that it is inhabited or uninhabited is matter of opinion. It has been proposed to discard from philosophical use these ambiguous expressions, and to divide knowledge, according to its sources, into matter of perception and matter of inference; and, as a cross division as to our conviction, into matter of certainty and matter of doubt. 'Subjective sufficiently is termed conviction (for myself); objective determination is termed certainty (for all). SEE KNOWLEDGE.