‘Reason’ itself does not lead us to form moral beliefs and we do not come to know of moral facts simply through a process of deductive reasoning.
Similarly, reason by itself does not lead us to believe that the causal outworkings of the world’s “nek minnut” will be regulated, or will accord with abstract mathematical ‘laws’ (however such laws may be hit upon).
Similarly again, reason alone does not lead to belief in God. (While a number of arguments to the existence of God are frequently derisively claimed to be “armchair philosophizing”, such arguments generally actually rely on empirical or experiential premises. The exception, perhaps, is the ‘Ontological Argument’, which having no formal background in modal logic, I shall not attempt to defend or assess, in any of its various forms – though I would suggest that even it derives any plausibility it has from our experience of the world.)
Instead, in each of these areas, we form entirely rational conclusions on the basis of our experience of the world. Sometimes we argue to a particular conclusion in each of these areas, but more often we seem to intuit a belief and work from there; examples are our beliefs that certain situations are either morally wrong or more valuable than others, our belief that the world will continue in generally the same way that it has done so far (on an everyday level as well as, more technically, when inferring or relying upon scientific laws – note, a realist committed to any of various explanatory virtues in science seems to rely on yet another inferential belief or set of beliefs), and our belief that God exists, formed perhaps when we see a stunning sunset, read the Bible, participate in communal sung worship, or reflect on the claims of Jesus.
Of course, our beliefs formed through our experience of life (or argued to inductively; perhaps inferred as the ‘best explanation’) are subject to defeaters, if such can be found. But let us not imagine that the ethicist, the scientist or the theist (or one who is all three, such as myself!) are delivering only the findings of reason when they recommend to us substantive beliefs about our world and its structure – or that reasonable people are limited to mere deduction from self-evident premises.
quod erat demonstrandum
While I’ve thought a bit about the link between these four things before, for emphasising the epistemic similarity between ethical and ‘causal-regularity’ or inductive claims (perhaps this could be broadened to the general case of rational inference rather than deduction) I am indebted to philosopher John Leslie’s 1979 book “Value and Existence”. Also very significant is Alvin Plantinga’s work on the rationality of belief, though I have not read as much of it as I would like to have done.