lf something fulfills its purpose, then we call that thing good, yes, but only because it is useful for producing desirable experiences. A triangle is a good triangle if it embodies “triangleness” well, but only because it is useful to be able to recognise and make good triangles. It can promote our understanding. It is much easier to see this in the case of a good ax, but it works for the triangle too.
So the ax and the triangle are good because by fullfilling their functions they can facilitate our happiness. This does not necessarily mean that these functions should be fulfilled in every possible scenario. We would not call it an evil against an ax to refrain from using it to murder someone. likewise, we would not call it an evil against our intellect to refrain from giving it te truth if the truth lead to some decline in happiness.
We call the thing good because it participates more in being. It is similar to calling a drawing of me a good drawing of me if it looks like me. This is done without reference to some function for which the drawing might serve as a means (as in, it’s good since it looks like me, which is good since it allows you to sell it to me and make money). In other words, analyzing the particular thing only without reference to anything else, a thing is a good thing insofar as it conforms to what that thing is meant to be, which is determined by its essence; it is good if it participates more in the being of the form which it instantiates by conforming more to the reality of the form. This gets at the notion that, in Thomistic philosophy, goodness and being are the same thing considered in different ways.
You are correct that not all functions should be fulfilled all the time. But this refers to a distinction between the good for one particular thing considered as itself only, and the good which takes into account the whole of things involved. Certain goods are above others. Hence it is an ontological good for the human reproductive faculty, considered as itself alone, to exercise itself and fulfill its natural end, but it is an evil for a human being to reproduce outside of the setting which is most conducive to the flourishing of the offspring, which is a lifelong commitment of marriage, since the good for the whole human being and the whole human species takes precedence over the good for one faculty in particular.
So the only reason that using an axe to murder someone is evil is because it contradicts the natural good of the other person, determined by his human nature, which is to grow, flourish, and know God and have beatitude in Him. Otherwise the axe exercising its function is good.
But there is no circumstance in which inculcating a definite falsehood into a human intellect can be conducive to the realization of the good for humans, since the human good involves realizing the individual goods of each human faculty in their proper place and measure. Thus it is good for us to eat, but not too much; it is good for us to reproduce, but not if we are not married. Taking falsehood into the intellect is not even simply not exercising the intellect; it is exercising the intellect, but in such a way that the intellect contradicts its own nature. It is good for us to exercise our intellect to know the truth; it can be acceptable for us not to know the truth on some occasions; it is never good, however, to accept falsehood with the intellect, since that is a violation of the good of the intellect, which is a part of the human good. So in doing evil to the intellect, one also does evil to the whole human being, of whom the intellect is a part.
)) ?