  There is a good explanation of the close connection seen by the rationalists between the epistemological and ontological orders, one that also accounts for their notable reliance on reason. It derives from their answer to what Leibniz called the grand metaphysical question: why is there something rather than nothing at all? There is something because there must be something; there cannot be nothing (and this way of putting it shows the ultimate debt of the rationalists to a tradition that goes back to Parmenides). Reality, or at least some part of it, has necessary existence, and that necessity is something like logical necessity. With this answer, a whole philosophical outlook falls into place. First of all, any significant role for sensory experience falls away, since what exists can be known a priori by logic alone. Causal connections tend to be viewed as logical connections; a principle of sufficient reason falls out which tends to be read as a matter of logical deduction. One result is that there is an impulse toward monism: if the ultimate cause must exist, then that for which it is the sufficient reason must also exist, and just how the two can be distinguished becomes problematic (again, the Parmenidean antecedent is clear).