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.
A similar change is care, cares, caress, but these are not nouns.
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