It is useful to assume objective facts. For example I’ve only ever seen things falling down. I have formed a model in my head and so far my predictions concerning gravity have always been useful in my life. You’re free to not believe in it but then your life will be more like a 1yo toddler in a baby’s high chair who has to try 500 times a day if the spoon will continue to fall on the ground the next time.
It’s kind of useless to say it’s a subjective and a matter of belief if things fall to the floor or to the ceiling or not at all. Or the spoon is a unicorn or you might not exist.