The author proposes a new method for rating textbook intelligibility. Under this method, the learners are given a textbook and a set of problems. The time needed to complete the problems is measured and the number of questions asked by the learners are counted. A syllogism test is also given in order to examine the learners' deductive inference abilities. The intelligibility of the textbook is based upon the time it takes each learner to complete the problems, the number of questions asked and the syllogism test scores. Experimental results using three different Objective-C computer language textbooks showed that textbook intelligibility ratings using this method have smaller deviations than do those which are based only upon the time it takes to complete the questions.