What is meant by "rational"?

What is the use of designating some formations of opinion as rational, while others (perhaps leading to the same results) are stigmatized as rules of thumb, or of authority, or as mere guesses? When we reason we set out from an assumed representation of a state of things. This we call our "premiss"; and working upon this, we produce another representation which professes to refer to the same state of things; and this we call our "conclusion". But so we do when we go irreflectively by a rule of thumb, as when we apply a rule of arithmetic the reason of which we have never been taught. The irrationality here consists in our following a fixed method, of the correctness of which the … method [itself] affords no assurance; so that if it does not happen to be right in its application to the case in hand, we go hopelessly astray. In genuine reasoning, we are not wedded to our method. We deliberately approve it, but we stand ever ready and disposed to reexamine it and to improve upon it, and to criticize our criticism of it, without cessation. Thus the utility of the word "reasoning" lies in its helping us to discriminate between the self-critical and uncritical formations of representations. If a machine works according to a fixed principle involved in the plan of it, it may be a useful aid in reasoning; but unless it is so contrived that, were there any defect in it, it would improve itself in that respect, then although it could correctly work out every possible conclusion from premisses, the machine itself would afford no assurance that its conclusion would be correct. Such assurance could only come from our critical examination of it. Consequently, it would not be, strictly speaking, a reasoning machine. —Charles S. Peirce, MS 831, p. 9f (1900)