That's why I don't agree with "don't show them thinking, show them doing". Of course, you'd have to show the thinking in ordinary words that a person of normal intelligence who hasn't had a lot of formal education might use. Proverbs might help; they could be part of the way your rationalist thinks, they could be part of the way /she explains things to others, and they could help your reader to remember it later, if/when they encounter similar situations in their own life. And you wouldn't call them a rationalist, that's a 'big word'. Other people would call them 'wise', and they themselves would probably say 'it's just common sense'.