In a post I wrote about Fides et Ratio a few months ago, I suggested that philosophical error is a kind of bondage (in contrast to the freedom that comes from the truth). Recently I was reading McInerny’s Praeambula Fidei, and came across a remark that is somewhat similar (which, in turn, he seems to have taken from St Thomas, who found it in Aristotle!):
The problem is posed at the outset of the De ente et essentia. Thomas cites the Aristotelian remark that a small error in the beginning causes maximal confusion eventually, and therefore he commends getting a correct understanding of being and essence [p. 189; emphasis added].
Here’s what Thomas said in De ente et essentia:
1. A small mistake in the beginning is a big one in the end, according to the Philosopher in the first book of On the Heavens and the Earth. And as Ibn-Sînâ says in the beginning of his Metaphysics, being and essence are what is first conceived by the intellect.
2. Thus, to avoid making mistakes out of ignorance of them, and to become familiar with the difficulties they entail, we must point out what is signified by the words “being” and “essence,” and how they are found in diverse things, and how they are related to the logical intentions, genus, species, and difference. [Source; emphasis added]
And lastly, from Aristotle’s De Caelo:
This being clear, we must go on to consider the questions which remain. First, is there an infinite body, as the majority of the ancient philosophers thought, or is this an impossibility? The decision of this question, either way, is not unimportant, but rather all-important, to our search for the truth. It is this problem which has practically always been the source of the differences of those who have written about nature as a whole. So it has been and so it must be; since the least initial deviation from the truth is multiplied later a thousandfold. Admit, for instance, the existence of a minimum magnitude, and you will find that the minimum which you have introduced, small as it is, causes the greatest truths of mathematics to totter. The reason is that a principle is great rather in power than in extent; hence that which was small at the start turns out a giant at the end. Now the conception of the infinite possesses this power of principles, and indeed in the sphere of quantity possesses it in a higher degree than any other conception; so that it is in no way absurd or unreasonable that the assumption that an infinite body exists should be of peculiar moment to our inquiry. The infinite, then, we must now discuss, opening the whole matter from the beginning. [Book I, 5 (271b5-18); emphasis added]
Ideas have consequences. When we build on a foundation that is fundamentally unstable (as error necessarily must be), that which we build upon it must necessarily be unsound itself. It’s better to recognize our mistake as soon as possible, so that we don’t have to rebuild, and so that no one gets hurt when the building comes crashing down. This isn’t true of literally every small mistake: as our authors above say, it’s the goofs at the beginning that cost the most in the long run.
Aquinas, Etc. 
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[…] Saint Thomas says, following Aristotle, a small error in the beginning leads to a big and difficult-to-correct one in the end, and one […]