These two paragraphs are taken from M. M. Sharif's A History of Muslim Philosophy, Chapter 25 1. Scroll down to page 3.
Ever since the criticism of ibn Sing’s doctrine by ibn Rushd who, among other things, accused ibn Sina of having violated the definition of substance as that which exists by itself, and of Aquinas who, although he adopts the distinction between essence and existence under the direct influence of ibn Sin, nevertheless follows ibn Rushd in his criticism, the unanimous voice of the Western historians of medieval philosophy has been to the effect that existence, according to ibn Sina, is just an accident among other accidents, e. g., round, black, etc. We have said that when ibn Sina talks of existence as an accident with relation to objects (as distinguished from essence) he just means by it a relation to God; it is, therefore, not an ordinary accident. Further, if existence were an accident, one could think it away and still go on talking of the object just as one can do in the case of other accidents and, indeed, in that case ibn Sina would have been forced to hold something like the Meinongian view held by many Muslim Mutakallims that non-existents must also "exist" in some peculiar sense of that word. But this is the very doctrine which ibn Sina ridicules. The whole discussion on this point can be found in the article referred to in note No. 5 of this chapter. Here we give only one passage where our philosopher criticizes the view of those who hold that a non-existent "thing" must, nevertheless, "exist" in some sense so that we can talk about it. He says (K. al-24.iffi', "Met." I, 5), "Those people who entertain this opinion hold that among those things which we can know (i. e., be acquainted with) and talk about, are things to which, in the realm of non-being, non-existence belongs as an attribute. He who wants to know more about this should further consult the nonsense which they have talked and which does not merit con-sideration." Indeed, according to ibn Sina, the ideas of existence and unity are the primary ideas with which we must start. These underived concepts are the bases of our application of other categories and attributes to things and, therefore, they defy definition since definition must involve other terms and concepts which are themselves derived (ibid., I, 5).
It will be seen that this problem now is not a metaphysical one but has to do with logic. Ibn Sina has attempted to give his own answer to the question : How is it possible that we can talk of non-existents and what do these latter mean ? His answer is that we can do so because we give to these objects "some sort of existence in the mind." But, surely, our individual images cannot constitute the meanings of these entities for the obvious reason that when we talk, e.g., of a space-ship, it must have an objective meaning. It is, nevertheless, true that ibn Sina has seen the basic difficulty of the logic of existence. And our modern logic itself, despite its superior techniques and some valuable distinctions, seems nowhere nearer the solution. It has tried hard to contend that whenever I talk of a space-ship, although none exists, I am not talking of a "thing," of an individual object, but only of a generic object or a conglome-ration of properties. But is this really so ? Is it absurd to say that the "individual space-ship I am talking of now has this and this property" ? Besides, the crux is the phrase "conglomeration or set of properties"—what is it to which they belong and of which I profess to be talking ?
In the first paragraph, it is said that Avicenna is ridiculing people who are trying to talk about non-existent beings, and in the next one, he goes on to explain this. What exactly is he talking about here?