Music theory is not a bunch of knowledge sent from heaven. Music is an observer of human history from the beginning of the creation. First times, there were percussions, wind instruments. Since the beginning of creation, music was not solely an art, but was also a science derived from nature.
Musicians founded a relationship between the notes kept the audience focused to their music. For instance, we might not be able to explain it with the music theory. But our ears know that there is something "strange", something "unexpected", something "absurd" is happening when F and G is played with C#.
One of the first impulses we get from the outer world is heart beats of our mother when we were fetus :) The birds harmonize together. The leaves harmonize with the wind. They all happen in harmony. This harmony in nature, what our ears are used to listen since the day we were a zygote, what is the tendency of the ears are documentated in the name of music theory.
All in all, this is not a memorized knowledge. It is documentation of human history and science.
C → D (WHOLE) E → F (HALF)
W + H = Major Third | H + W = Minor Third
Major Third + Minor Third = MAJOR Triad (C-E-G)
Minor Third + Major Third = MINOR Triad (E-G-B)
There is no certain knowledge for it. Triads are documentated but there are power chords (5), there are suspended chords (sus4, sus2), there are chords with four notes (7th). Build with any interval!