The word "opinion' is in the same family of words to which "knowledge" belongs. Both words refer to acts of the mind that can be objectively true or false. But there is an important difference. False knowledge is self-contradictory. If a statement is false, it does not express our knowledge. But a statement of opinion can be either true or false.
To take account of this difference between knowledge and opinion, it is necessary to regard opinions as only probably true or false, whereas knowledge is true beyond the shadow of a doubt. It has certitude. Self-evident truths, truths the opposites of which are unthinkable, are invariant truths. Moreover, those things which can be validly deduced from such truths are similarly beyond the shadow of a doubt. They have certitude.
The Greek language had two words that we can translate into English by referring to the assurance we have about their truth. One was the word episteme. The other was the word doxa.
Episteme from which we get the English word "epistemology, stands for truths that have invariable certitude for us. Doxa is the word from which we derive the word "orthodox." That English word means right opinion -- opinion of which, at a given moment in time and in the light of all the then available evidence and reasons, we feel assured either beyond a reasonable doubt or at least by a preponderance of the then available evidence and reasons. But unlike knowledge in the strongest sense of the of that term episteme, doxa is never beyond the shadow of a doubt, it includes all doubt able truths. The assurance we have about their truth is always determined, at a given time, by the character of the evidence and reasons we then have at our disposal.
Setting aside, for the moment, the very few propositions that are either self-evident or demonstrable in terms of valid deductions from self-evident truths, all the propositions of history, of the empirical science, and of philosophy fall in the sphere of doubt. They are all instances of doxa not episteme.
It is only the last few centuries of modern time that philosopher have bewildered themselves with the problems of epistemology. If they had concerned themselves with the opinions of historian, empirical scientists, and philosophers, their inquires would have been much more fruitful.