From The Collaborative International Dictionary of English v.0.48:
Rationalize \Ra"tion*al*ize\ (r[a^]sh"[u^]n*al*[imac]z), v. t. 1. To make rational; also, to convert to rationalism. [1913 Webster]
2. To interpret in the manner of a rationalist. [1913 Webster]
3. To form a rational conception of. [1913 Webster]
4. (Alg.) To render rational; to free from radical signs or quantities. [1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Rationalize \Ra"tion*al*ize\, v. i. To use, and rely on, reason in forming a theory, belief, etc., especially in matters of religion: to accord with the principles of rationalism. [1913 Webster]
Theodore . . . is justly considered the chief rationalizing doctor of antiquity. --J. H. Newman. [1913 Webster]