Sartre thought that we feel anxiety when we appreciate the responsibility we bear whenever we act—when we realize that, ultimately, there is nothing out there, including the ethical systems we are born into, that backs up our choices, and nothing that guarantees that they will be the right ones. Dogs and cats, presumably, do not know this feeling. Anxiety is the price tag on human freedom.
There’s no certainty but uncertainty. Beliefs are really just bets with high probability of the truth because that’s how things usually go.