Irenaus and biblical interpretation

While in seminary and even before that, when I would read theology or maybe a Biblical commentary on a certain book or passage, one principle of Biblical interpretation always came up. Always interpret obscure or difficult passages in light of clear ones. I was a bit surprised to hear that it was Irenaus who came up with this in the 2nd century of so.

WHat is this principle? Don't try to build a theology around a verse or passage which can be difficult to interpret or understand. Instead focus on a verse or passage like it wich is much clearer to understand. Difficult passages lead to difficult and multiple interpretations.