How and when should I correctly use the article "the" after a preposition, pron. & det.?

Which sentence is correct looking at the use of the article "the" in bold?

• By a way of illustration, let’s say that the Keynesian paradigm identified «full employment» as the core subject of its macroeconomic theory and policy..

• By a way of illustration, let’s say that Keynesian paradigm identified «full employment» as the core subject of its macroeconomic theory and policy..

For the sake of clarity, include the before the subject phrase. Paradigm is a singular noun that should be preceded by an article.

In fact, the optional word in your sentence is that. It can be removed without impacting the meaning of the sentence.