Which is the fourth source used to derive Islamic knowledge?

After the Quran, the Sunnah, and the Ijmaa’, the ‘Aql, or sound reasoning can be used as a fourth source to derive Islamic knowledge. Again, of course, this would be sound reasoning based on what is known or available from the aforementioned 3 sources.

The first three sources show the importance of sound knowledge as a source for all actions. But the Aql is the first of two levels of personal responsibility, even when the first three sources are either unavailable or limited.

Linguistically, the root word ‘Aql’ has to do with a preventer or inhibitor, and just the same, a human’s ‘Aql or sound reasoning is gifted to him as a means to prevent him from falling into harm's way.