Puzzle ZOYC 

Workings

Hint
The code has just four letters, A, B, C, and D. Each letter encodes 5 different letters.

Answer
I think, therefore I am.

Reasoning
A represents a, b, c, d, and e.
B represents f, g, h, i, and j.
C represents k, l, m, n, and o.
D represents p, q, r, s, and t.
E represents u, v, w, x, and y.