Simplifying drastically, fully general lossless data compression is impossible because of a simple No Free Lunch theorem. To achieve compression in practice, a

Simplifying drastically, fully general lossless data compression is impossible because of a simple No Free Lunch theorem. To achieve compression in practice, a compressor must exploit empirical regularities in the data. For example, PNG can achieve compression on most real world images because it exploits a regularity that they exhibit, which is that adjacent pixels tend to be highly correlated. So lossless data compression is about documenting and describing empirical regularities in real world observations; exactly the same as empirical science.