Wittgenstein asserts that claims like “here is a hand” or “the world has existed for more than five minutes” have the form of empirical propositions but that in fact they have more in common with logical propositions.
That is, these sorts of propositions may seem to say something factual about the world, and hence be open to doubt, but really the function they serve in language is to serve as a kind of framework within which empirical propositions can make sense.
In other words, we take such propositions for granted so that we can speak about the hand or about things in the world—these propositions aren’t meant to be subjected to skeptical scrutiny.
At one point, Wittgenstein compares these sorts of propositions to a riverbed, which must remain in place for the river of language to flow smoothly.
The river-bed is only relatively stable with respect to the water flowing over it, because it is worn away with time, and shifts its course, sometimes the silt that is settled at the bottom of the river bed may be disturbed and become part of the river.
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