Clearly, mere congruity does not justify belief.

In the proposition _Water rusts iron_, the concepts _water_, _rust_ and _iron_ may be congruous, but does any one assert their connection on that ground?
In the proposition _Murderers are haunted by the ghosts of their victims_, the concepts _victim_, _murderer_, _ghost_ have a high degree of congruity; yet, unfortunately, I cannot believe it: there seems to be no such cheap defence of innocence.

Now, Mill held that Logic is concerned with the grounds of belief, and that the scope of Logic includes Induction as well as Deduction; whereas, according to Hamilton, Induction is only Modified Logic, a mere appendix to the theory of the "forms of thought as thought." Indeed, Mill endeavoured in his _Logic_ to probe the grounds of belief deeper than usual, and introduced a good deal of Metaphysics--either too much or not enough--concerning the ground of axioms.

But, at any rate, his great point was that belief, and therefore (for the most part) the Real Proposition, is concerned not merely with the relations of words, or even of ideas, but with matters of fact; that is, both propositions and judgments point to something further, to the relations of things which we can examine, not merely by thinking about them (comparing them in thought), but by observing them with the united powers of thought and perception.

This is what convinces us that _water rusts iron_: and the difficulty of doing this is what prevents our feeling sure that _murderers are haunted by the ghosts of their victims_.