I see there are multiple good answers already, but I'll just add my 2c - which answers from a different angle, sort of (though partially similar to some of the

I see there are multiple good answers already, but I'll just add my 2c - which answers from a different angle, sort of (though partially similar to some of the other answers). It may help, if you understand recursive calls to functions, and an imperative language like Python - the examples are in Python: Recursive computation of simple functions: https://jugad2.blogspot.in/2016/03/recursive-computation-of-simple_2.html https://jugad2.blogspot.in/2016/03/recursive-computation-of-... The first example shown is for computing the length of a list, recursively. Edit: To make it clear, my example uses an imperative approach, while your Prolog example uses a declarative approach. As the excellent comments by carapace and kevhito show, the two approaches are different. Just that my example may help you to get the solution used. But in Python, we tell the interpreter how to compute the length of a list. In Prolog, it is sort of as though we tell the interpreter what we want it to find out (a goal), and give it some facts / properties and rules to help it do that. It then does that, using its inference engine. HTH.