Full Idea
When the only rule of inference is Modus Ponens, the branches of a tree proof soon spread too wide for comfort.
Gist of Idea
A tree proof becomes too broad if its only rule is Modus Ponens
Source
David Bostock (Intermediate Logic [1997], 6.4)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.263
Related Idea
Idea 9394 MPP: Given A and A→B, we may derive B [Lemmon]