Science is different from other epistemologies in that it is founded, ultimately, on empirical observation. Where philosophy may rely on logic, and the humanities on texts, sciencists draw their conclusions from observations conducted under controlled conditions. Scientists, like others, construct hypotheses and theories, but these theories must be tested against observation and experimentation in order to have validity. To some extent, this is true of other disciplines. Historians, for instance, also develop theories, which are subjected to rigorous inquiry based on the sources and on their internal logic. But science is fundamentally based on observation. Many philosophers have questioned just how certain the conclusions of science are, noting that, for example, the fact that something happens again and again (like a falling object, for example) does not mean it is inconceivable that a different result could occur. But the crucial thing to understand is that science then adjusts its theories, offering hypotheses to explain the new phenomenon. These hypotheses are then tested against further observations.