Soul is to explain some physical phenomena for which no physical cause is observed. This physical phenomena in question is "decreasing entropy of a living cell". There is no physical difference between a living cell and a dead cell. Had there been any physical difference between a living cell and a dead cell, the phenomena of 'decreasing entropy in a living cell' could have been attributed to that physical difference. But no such physical difference is observed. So, the logical conclusion to explain the physical phenomena of "decreasing entropy of a living cell" is some non-physical existence. Soul is just the name given to this non-physical existence.