Foundationalism is the mistaken notion that some certain knowledge comes to us ready-made, and does not depend on anything else. One common sort involves what Wilfrid Sellars called the Myth of the Given.
Certainty comes from proof. A mathematical construction is certain. Nothing in philosophy or ordinary life is like that. There are many things we have no good reason to doubt, but without proof, that still does not make them certain.
In life, high confidence is all we need. Extreme skepticism is refuted by experience. It is not possible to live a life without practical confidence in many things.
Truth, however, is a result, not a starting point. It must be earned. There are no self-certifying truths, and truth cannot be an unexplained explainer.
In philosophy, we have dialectical criticism or analysis that can be applied from any starting point, then iteratively improved, and a certain nonpropositional faith in reason to get us going. All we need is the ability to question, an awareness of what we do not know, and a little faith. We can always move forward. It is the ability to move forward that is key. (See also Interpretation; Brandom on Truth; The Autonomy of Reason.)