ثَلَاثَةٌ: الْحَوَاسُّ السَّلِيمَةُ، وَالْخَبَرُ الصَّادِقُ، وَالْعَقْلُ بِحُكْمِ الِاسْتِقْرَاءِ
By an inductive classification, the three principal means are sound senses, truthful report, and reason.
Sound senses apprehend sensible matters when the faculty and conditions are suitable. Truthful report conveys information whose truth has appropriate grounds. Reason handles concepts, judgments, and valid inference.
Seeing an object oneself, learning through trustworthy testimony, and drawing a conclusion from established premises are distinct routes. The purpose of naming them is not merely to memorize a triad but to understand what each contributes and where each needs qualification.