We use them everyday - to build things, buy things, gamble and grow. Yet we use numbers without thinking about how peculiar they are.
We first learnt to count by saying: '3 apples and 2 apples makes 5 apples' and we could check this by actually taking physical apples and grouping them.
And then we advanced to: '3 apples plus 2 apples equals 5 apples'.
But can you remember at what point you made the huge leap and forgot about apples and instead said: 3+2 = 5?
It happens so subtly we can't remember, but it is a truly remarkable leap of faith - that what's true for apples is true for anything we want to count and is still true when we're not associating the figures 2 and 3 with any entity. We just slipped from apples to abstract without noticing!