The Ismaelis Argue That Humankind Requires an External Guide

This idea that they have that humankind requires a guide not just the prophet but also Ali following the prophet Muhammad and then the imams. What is their argument for this? Because this is a position that was attacked by philosophical thinkers Al-Bukkah Razi and Al-Kazali to people who are usually put in the same class together. And it seems like the Ismaelis are making a very bold epistemological claim here which is that for most of us so everyone other than one of these imams or prophets it is impossible to achieve knowledge on our own. Do they have any kind of argument for convincing us of that?