Aristotle explains most of his predecessor’s opinions on nature by saying that they fail to distinguish what is said simply and without qualification (simpliciter) from what is said in a certain way, or with qualification (secundum quid). When we speak of a living thing, without qualification, we are taling about some animal or plant; but we can also call a heart or a lung a living thing, if we add the qualification that it is a part of an animal. It is important to distinguish these senses because both kinds of statements can be true, and even true always, but the word does not have the same meaning in these two cases, and so if we confound the senses we will commit the fallacy of equivocation.
Aristotle, following the lead of Plato’s Sophist, noted that it is false to say that “what is not a being is non being”. His reason was that being simply speaking referred to a substance, but it does not follow that if something fails to be a substance that it is nothing at all. This accords with our everyday experince- if you ask someone to name off a list of “beings” he will list off a bunch of substances, because this is what the word being means without qualification. It does not follow from this, however, that shapes or relations or colors are nothing at all, and in this qualified sense such things do deserve to be called beings, but secundum quid.
Just as the distinction between simpliciter and secundum quid when applied to beings gives rise to the distinction between substance and accident; so too the same distinction applied to motion and becoming gives rise to the distinction between act and potency. The riddle of the ancients was set by Parmenides: if something comes to be, it does so either from what is, or from what is not, but if from what isthen it did not come to be, if from what is not, it cannot come to be. Therefore motion or coming to be is impossible. Aristotle’s answer is that it comes from neither of them, simply speaking, but only in an accidental sense. Properly speaking, what comes to be something is what was able to be that same something, insofar as it is able. The only reason to speak about ability at all is to indicate something that could come to be from the ability. Insofar as this ability is somehow distinct from what is absolutely nothing at all, it makes sense to call it being in a qualified sense, or secundum quid.
Just Thomism 
Linus2nd said,
January 5, 2014 at 1:44 pm
Thank you. I was wondering about the meaning in Aquinas’ Principles of Nature or Letter to Brother Sylvester, ” 1. Since some things can be, although they are not, and some things now are; those which can be and are not are said to be potency, but those which already exist are said to be in act. But existence is twofold: one is essential existence or the substantial existence of
a thing, for example man exists, and this is existence simpliciter. The other is accidental existence, for example man is white, and this is existence secundum quid. ”
So I see that it means, ” existence in another, ” or existence in a certain manner of speaking.
Monk, Francis Mary said,
October 2, 2016 at 10:58 am
Ave Maria, one needs to listen to Aquinas Thomas the Angelic Doctor with the soul ear and see him with the soul eye to be able to understand him. God bless.