First cause and Primary cause

First cause noun - An agent that is the cause of all things but does not itself have a cause.

Primary cause and first cause are semantically related in ultimate cause topic. In some cases you can use "Primary cause" instead a noun phrase "First cause", when it comes to topics like original cause.

First cause and primary cause are semantically related in ultimate cause topic. You can use "First cause" instead a noun "Primary cause", if it concerns topics such as original cause.