Beliefs are mental structures.

'Opinion' is a synonym for 'belief'. Opinions are not necessarily right nor necessarily true.

In "The Fixation of Belief," Charles Sanders Peirce discussed the nature of doubt and belief (published originally in Popular Science Monthly in 1877, also published later in Collected Papers, 1934, Volume 5, Book 2, Paper 4).

Peirce wrote:

Our beliefs guide our desires and shape our actions... The feeling of believing is a more or less sure indication of there being established in our nature some habit which will determine our actions. Doubt never has such an effect (5:371).

Doubt is an uneasy and dissatisfied state from which we struggle to free ourselves and pass into the state of belief... (5:372).

The irritation of doubt causes a struggle to attain a state of belief. I shall term this struggle Inquiry (5:374, bolding added).

Peirce identified four methods of fixation of belief:

  1. Tenacity
  2. Authority
  3. Agreeablenesss to Reason (a Priori)
  4. Science