In addition to an argument being both valid and sound, an argument also needs to have justification or warrant for each of the premises that it presents (if the premises aren’t just obviously true). Something obviously true would be something like, “A triangle has three sides”. This is simply true by definition, an analytic truth.
If we have good reasons to accept all the premises of a deductive argument, then the conclusion necessarily follows once we grant the truth of each premise. We don’t have to know each premise with certainty. In logical form, the premise is either true or false, but us knowing is a different story. Most Philosophers agree that we don’t have to demonstrate a premise to be true beyond a shadow of a doubt or all possible doubt. In fact, as long as the affirmation of the conjunction of all the premises is significantly more plausible than the negation, we are warranted in (outright) believing the conclusion. Some people debate on the exact percentage but let’s say 70% is a reasonable proposal for full belief in a claim.
We don’t have to know each premise with certainty because we don’t know very much with certainty if you think about. Epistemologists have come to recognize that it would be impractical and unreasonable to demand that most or a lot of our beliefs be based in certainty. So the lesson is that if someone tells you that you need to demonstrate a claim with certainty, you can go ahead and call B.S.