Professor: It has been argued that freedom of thought is a precondition for intellectual progress, because freedom of thought allows thinkers to pursue their ideas, regardless of whom these ideas offend, in whatever direction they lead. However, it is clear that one must mine the full implications of interrelated ideas to make intellectual progress, and for this, thinkers need intellectual discipline. Ttherefore, this argument for freedom of thought fails. The conclusion drawn by the professor follows logically if which one of the following is assumed? ['Without intellectual discipline, thinkers can have no freedom of thought.', 'Thinkers who limit their line of thought to a particular orthodoxy are hindered in their intellectual progress.', 'Freedom of thought engenders creativity, which aids the discovery of truth.', 'In societies that protect freedom of thought, thinkers invariably lack intellectual discipline.']