Reasoning works by means of thinking along a line of reasoning. A line of reasoning is a line where each point in the line is a reason-computation that takes the conclusions from the previous point as that point’s premises, reasons what must be true if all those premises are true, forms conclusions, and then hands off those conclusions to the next point in the line to be the premises for the next point. Lines of reasoning prove things, but lines of reasoning must have their premises checked, especially if the conclusion turns out to be false or to resolve to a contradiction.
To check a line of reasoning, you start at the conclusion and work your way back up to the start, confirming that each premise which was reasoned from was correct, and a key task is to look for hidden premises, things which were assumed for the analysis without awareness or without conscious thought and deliberation choosing to accept those premises before they were adopted. Assumptions, or what you take for granted but which turns out to be not true, for examples.
To check a premise, you can assert its truth and see if it collapses to or reduces to an absurdity if it was true, and if it does then it is false; you can test it against empirical experience and evidence and observation, to see if it is false; or you can ask “why? Why would this be true?” and, if the reason behind it is incorrect or does not make sense, try again without it.
Reasoning from only true premises, if the reasoning is valid and sound and rational, produces conclusions which must be true, because the truth of the premises will cause the truth of the conclusion, because the conclusion is that which must be true if all the premises are true according to logic, but if there is at least one or more false premises, the conclusion will be false, and, if there is a premise or premises whose actual truth or falsehood you do not know, then the line of reasoning is in the condition of being able to be true or false, and you won’t know which it is.
Reason from 100% true premises is pure reason. Reasoning from false or dubious premises is corrupted reason. Pure reason produces knowledge of the truth. Note that a premise, as I see it, can be anything that is reasoned from: a perception or observation, a concrete thing, an abstraction or essence of being, a fact, a principle, a belief, an opinion, knowledge, etc. If your reasoning resolves to a contradiction or a conclusion you know must be false, take Ayn Rand’s advice: Check your premises!
