Some men believe their own Opinions no less firmly than others do their positive Knowledge.

Aristotle (384-322 BC) Greek philosopher

Nicomachean Ethics [Ἠθικὰ Νικομάχεια], Book 7, ch. 3 (7.3) (c. 325 BC) [tr. Chase (1847)]
    (Source)

Alternate translations:

Some men put no less faith in their own uncertified opinions than do others in the verified truths of science.
[tr. Williams (1869), sec. 127]

For some people are as strongly convinced of their opinions as others of their knowledge.
[tr. Welldon (1892)]

Some people have just as strong a belief in their mere opinions as others have in what they really know.
[tr. Peters (1893), 7.3.4]

Some men are no less convinced of what they think than others of what they know.
[tr. Ross (1908)]

Some men are just as firmly convinced of what they opine as others are of what they know.
[tr. Rackham (1934)]

Some people have no less conviction about that they believe than others do about what they know scientifically.
[tr. Reeve (1948)]

Some men are just as sure of the truth of their opinions as others are of what they know.
[tr. Thomson (1953)]

Some men are no less convinced of their opinions about things than others of the things they know.
[tr. Apostle (1975)]

There are some people who have no less confidence than others hav ein what they know.
[tr. Thomson/Tredennick (1976)]

Some are no less convinced of what they opine about than are other people of what they know.
[tr. Bartlett/Collins (2011)]