Our friend, Plato, believes in dualism, or the notion that there exists two different aspects of things, the material and the immaterial. The material consists of the body and all things within the realm of sensory experience, while the immaterial consists of things like the intellect and all things that are, as Plato terms it, invisible. It is the latter category that Plato says is superior and he bases the understanding of truth within the realm of the immaterial. The reason for his escalation of the immaterial is the fact that the senses are fallible, which makes them incompatible with truth.
The problem with the view that the intellect is superior to the senses because it is infallible is that, as anyone who has spent time conducting arguments in logic would account, the intellect is just as prone to making mistakes as the senses. Indeed, the ability of false premises to lead to a true conclusion and form a valid logic statement shows that logic, which is based in pure reason, is fallible in it's own accord. If, within logic, falsity always lead to falsity and truth always led to truth, then this would hint toward a system of infallible intellect, but this is not the case.
Instead we should acknowledge that we are faced with basing understanding on the flawed and imperfect systems (intellect and experience) and can know nothing for certain (since we are working from systems of understanding that are themselves imperfect, everything is open to questioning). With this starting point we can then work toward an approach to truth in which we work not toward absolute conclusions but conclusions of likelihood.