Popper argued for falsifiability and opposed this to the intuitively similar concept of verifiability. Verifying the claim "All swans are white" would require assessment of all swans, which is not possible in any theory that has a reasonable empirical interpretation. In contrast, the single observation of a black swan is sufficient to falsify it. Moreover, even if a black swan was in principle impossible due to a fundamental law of biology, it would normally still remain a "possible observation" in the conventional empirical language.