You can’t be certain about anything in the real world. But some things are far more likely than others. It is more reasonable to believe some things than others.
We should base our decisions, actions, and explanations, on what is most reasonable, and not expect to have to apply some criteria of ‘absolute’ truth.
In broad terms we can consider something reasonable if: (a) it is consistent with all the known facts, (b) it can in principle be proven false, (c) it is the most general, useful, and simplistic consistent with (a) and (b).
Something should be considered true so long as it is the best to be had. It becomes false as soon as it can be bettered.
Ensure that truths which are determined by argument do not involve any fallacies in the argument. There are a large number of well known fallacies which frequently occur in everyday life. They are particularly prone to occur in verbal arguments.
Examples of fallacies: (a) mistaking things that are different for things which are mutually exclusive, (b) believing only two states exist when in reality there is a continual scale, (c) where one part of an argument is true only if ‘some’ appears in front of it, but the argument then proceeds as if ‘all’ had appeared.
Never ignore evidence or facts which contradict what is currently believed. By all means question the evidence/facts, but never dismiss it solely because it contradicts existing facts/beliefs.
Just because we have facts/evidence that contradicts what we currently believe doesn’t mean we should abandon what we currently believe. Usually we will need to modify it. After all, any new truths/beliefs must account both for the new facts/evidence and also for all the facts/evidence accounted for by our previous truths/beliefs.
People often believe what they would like to be true rather than what there is evidence for being true. Sometimes people will hold onto beliefs long after there is clear evidence to the contrary.
Truths are sometimes inconvenient. This doesn’t make them untrue. We often need to better distinguish truths from the conclusions we draw from them.
Most concepts and truths have a fuzziness about them. We don’t need to precise about them most of the time, though sometimes we do for the particular purpose in hand.
Just because someone has different beliefs to yourself doesn’t mean you should try to change those beliefs.