From what I have come to understand, the events A and B are considered independent for purposes of probability theory when
$$ p(A \cap B) = p(A) \cdot p(B) $$
Now, supposing I flip two coins. I write down the probabilities for the joint outcomes as $\left(\frac{1}{4}, \frac{1}{4}, \frac{1}{4}, \frac{1}{4}\right)$ and see that the individual coins are independent. But how do we know those probabilities? Well, the first coin could come up either heads or tails with probability $\frac{1}{2}$, then in the case of heads we know that the probabilities for the second coin are still $\frac{1}{2}$… but this seems to be assuming $p(A|B) = p(A)$, which is mathematically just a rearrangement of the above formula, so this feels like circular reasoning.
So, what's the explanation? Why does theoretical independence correspond to practical independence, in a not-apparently-circular way?