Augustus
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So if one person says to me "gods exist" I must believe that and if a second person says "gods don't exist" a minute later I must believe that too? At the same time?
Only if your brain works remarkably slowly. The secondary process of correction often occurs a fraction of a second later.
You comprehend propositions as being true and, to change this, you must make a secondary corrective action by comparing what you comprehended with other knowledge and beliefs. The alternative is that we comprehend something neutrally and have a 'lack of belief' until we choose one way of the other.
Thinking of it in terms of believing probably makes people think of a much more conscious and drawn out process, things are comprehended as true unless corrective action is taken. It's not really an issue on topics we are familiar with due to correction being automatic.