In a future essay I will outline the case that the cause of certainty is the interior action of the Holy Spirit. I will also argue that the Holy Spirit can lead the person to confirmations – at the level of reason – that it is the Holy Spirit at work, and that this causal attribution is not a cognitive error. Thus I make a distinction between the cause of profound conviction, which is often too deep to express in words, and signs (special graces) which confirm the nature of the cause.
I will draw upon this paper, which is available on the Resources page:
Block, Benjamin M. (2015). The Balance of Faith and Reason: The Role of Confirmation in the Thought of St Thomas Aquinas. Studia Gilsoniana 3 (September): 209–28.