Are you perhaps looking for the expression, "take for granted"? As in, "We take for granted that the sky is blue, sunshine is warm, and snow is cold."
There is also the word "assumption." This has a more argumentative tone. It basically means, "a belief that you have no evidence for and/or can't prove, but which you believe to be true anyway." There are 2 main types. Descriptive assumptions are beliefs about how the world is: "the sky is blue", but also more sinister and discriminatory ones like the famous, "Mexicans are all dirty murderers and rapists." Then prescriptive (a.k.a. "value") assumptions are moral and/or ethical ones: stealing is bad; we ought to all donate our organs; human life should always be valued above the lives of other organisms.
Warning: In everyday conversation, "assumption" usually has a negative connotation; it means you're being called out for a mistaken belief, or calling someone else out on a belief that isn't necessarily true. E.g. "Stop assuming things," means, "Stop making up beliefs/interpretations you don't have evidence for."
When you use "take for granted (that)," you specify what the belief is: "Having safe tap water is taken for granted in this city." "We (can) take it for granted that the sun will rise tomorrow."
When we use "assumption," it replaces the description of the belief: "You made an assumption about the tap water's safety." You can, of course, turn "assume" into a verb to describe the belief: "We assume the tap water is safe." "We (can) assume that the sun will rise tomorrow."