Change refers to a difference in a state of affairs at different points in time. Although it is a familiar experience, an analysis of change provides subtle problems which have occupied philosophers since the Presocratics. Heraclitus is the first philosopher known to have directly raised such issues, with aphorisms such as "one cannot step into the same river twice". The Eleatics were particularly concerned with change and raised a number of problems, including Zeno's paradoxes, which caused them to go as far as insisting that change was impossible, and that reality was one and unchanging. Later philosophers would reject this conclusion, instead developing systems such as atomism in attempts to circumvent the Eleatic problems. In the modern era, some of these problems would enter the domain of mathematics, with the development of calculus and analysis. These developments were regarded by some as solving problems of change, but others maintain that philosophical issues persist.