The word “object” occupies a prominent place in philosophy, but it is seldom scrutinized in any depth. What is an object exactly? On the face of it the word has two uses or meanings: it may be used as a descriptor of a certain type of entity in contrast to other types, as with the distinction between objects and properties; or it may be used to indicate a relation between a thinker and what she or he is thinking about, as in the phrase “object of thought”. In this latter sense we speak of someone as the “object of attention”, or of “objects of experience”, or of “my object in doing such and such” (goal, aim), or of the “direct object” of a verb, or of “nonexistent objects of hallucination”. Objects in this sense need not exist: a mental state can have an object—it can be about something—even if that object doesn’t exist. Fictional objects are objects of thought that don’t exist. It is not always clear what sense is intended, or whether the duality of uses is even recognized. The OED gives this for “object”: “a material thing that can be seen and touched”, and “a thing external to the thinking mind or subject”. The confusion is evident here. Must an object be a material thing? Why make reference to what can be sensed? Why just sight and touch? Why must an object be external to the mind—aren’t there mental objects, and isn’t the self an object too? The philosopher seems to want to designate a very broad ontological category that exists independently of perceiving minds and makes no reference to mental acts (as with Frege’s and early Wittgenstein’s use of “object”), but also to speak of mental acts in which things are “posited” or “apprehended”. The word “object” shuttles between these uses rather indiscriminately, now meaning one, now the other. Linguistically, it’s a mess.
What should we say about these two ways of using “object”? One view would be that we have a straight ambiguity analogous to “bank”—the same sound or mark just happens to have two quite distinct meanings. But this is hardly plausible: surely the two meanings are connected in some way. It is natural to suppose that one use is primary and the other derived: “object” as a type of entity or “object” as a way of talking about mental acts. Suppose we take the first use as primary: then “object of thought” comes to mean something like, “object (first sense) that happens to be thought about”. This has the problem that not all objects of thought exist, but all objects in the first sense do: so “object of thought” can’t mean “existent thing that someone happens to be thinking about”. The whole point of the phrase “object of thought” is to make room for mental acts about nonexistent things. Also, objects of mental acts need not be objects in the first sense, since mental acts can be about properties, facts, moral values, etc.—these are their “objects”. More promising is the idea that the mental act sense is primary: an object is what is or can be an object of thought (experience, emotion, intention, etc.). This is supported by the etymology of the word: it derives from a Medieval Latin word objectum meaning “thing presented to the mind”. Thus whenever “object” appears in philosophical discourse it means, “object of thought”. There are two problems with this. The first is that not all objects are objects of thought, since some are not thought about—e.g. remote galaxies and their constituent parts. The second is that it is clear that not all uses of “object” by philosophers can be so paraphrased: for example, Wittgenstein’s use of “object” in the Tractatus. At 2.01 he says: “A state of affairs (a state of things) is a combination of objects (things)”; at 2.014 we have “Objects contain the possibility of all situations”; and at 2.02 we read “Objects are simple”. He clearly doesn’t mean the word “object” in the mental act sense, and indeed he slides between “object” and “thing” without giving any notice of a shift of sense. Similarly, when Frege announces that truth-values are objects he doesn’t mean that they are things that we think about; he means to be making an ontological remark. He is distinguishing between objects and concepts (functions, in his system)–as Wittgenstein is distinguishing between objects and facts. So it is wrong to suppose that this use of “object” can be paraphrased by invoking the other use. What is to be done?
One thing we can do is firmly mark the distinction and pay due attention to it. We could in principle simply ban one use so as to clean up the language and not be bamboozled by it: we could, say, reserve “thing” for objects in the first sense and continue to use “object” only in the second sense (thus respecting etymology). I am not against this idea, though it would be pretty impractical given the entrenchment of the first sense (but in an ideal language…). It has the advantage of preventing a certain kind of misguided objection to Meinong, namely the sneering riposte, “But nonexistent objects of thought are not objects”—as if Meinong is contradicting himself. Sure they aren’t, but the whole point of Meinong’s philosophy is to distinguish between what we think about and what really exists (the former having a special type of “being”). Let’s agree that objects of thought (“intentional objects”) are not objects in the sense in which Frege and Wittgenstein speak of objects—whoever said they were? We can continue to investigate the nature of objects of mental acts, observing (say) that such objects need have no mind-independent existence, or that they lack ontological depth, or that they violate the law of excluded middle. This verbal recommendation would certainly put paid to a lot pointless squabbling. I suspect myself that “object” in the ontological sense is an example of creeping semantic shift, whereby the original word (objectum) was extended to anything that exists whether thought about or not. This can lead to an unspoken idealism according to which everything that exists is really intrinsically a mental object—for everything is like an object of thought. We conceive of everything as if it were something thought about—an object of apprehension. But we need to make a firm distinction between an object of thought and a real existing object—though we do sometimes think about the latter. Clarity would be served by calling objects in the first sense “things” and reserving “object” for objects of thought. Thus we might ask how many things (of a certain kind) exist in a given room, and we might ask how many objects came before my mind in that room during a certain time interval: these are quite different kinds of question. I might have three cat-things in my room but have hallucinated a dozen cat-objects. It only invites confusion to report that there were three objects of a certain type at a given time in a particular place but a dozen objects of the same type at the same time (three actual cat objects but a dozen object-of-thought cats). In any case, we do well to be clear about the distinction and be constantly on guard against confusing the different uses of “object” that now infect our language.
It may be wondered whether we really have a notion of object in the ontological sense. Objects of thought are conceptualized things, unitary and well defined, but are things considered independently of the mind similarly unified and well defined? We might toy with the idea of a “natural object”—one that is unified by nature, as it were. Maybe there is such a thing—organisms provide plausible examples—but the principle of unification is not like that imposed by the mind. Nature is not as Gestalt as the mind is, and it obeys different rules of unification. I conceive of you as a unitary person, but nature might regard you as just an assemblage of parts. Is Mount Everest really the natural unity that it is represented as being in human thought? One can at least sympathize with those philosophers who have found in nature only undifferentiated stuff not individuated objects; they have an exaggerated response to the insight that the world is not as cleanly segregated objectively as human thought represents it as being. But objects of thought enjoy a kind of ideal unity, since they have no being beyond what the mind invests in them. They are the real objects not the amorphous and pointless stuff that populates the mind-independent universe. In other words, natural objects are not as clearly defined as intentional objects, which enjoy a kind of conferred unity. The latter are human objects, fitting human purposes; the former are just products of natural forces that lack conceptualized unity. If we project the unity of intentional objects onto nature, we endow nature with a mind-centeredness it doesn’t strictly deserve. We engage in a kind of unintended idealism. This is why I am not averse to restricting the word “object” to objects of mental acts and referring to everything else with the word “thing”, from which any suggestion of presentation to the mind is expunged. Thus the objective world is not strictly speaking composed of objects, though it is composed of things that bear a certain complex relation to objects of thought. 
I am struck by the fact that “object” in the mental act sense has so few synonyms or even metaphorical expressions (perhaps this is why Husserl had to introduce neologisms like “noema”). The closest word I can think of is “target”—what the mental act “aims” at. The mind targets its objects of thought. But even that is jejune and unhelpful: what kind of targeting is this, and with what weapon? We are stuck with the locution “object of” expressing a relation whose nature remains obscure. The word “object” here just seems to mean, “what is apprehended”—what is on the receiving end of thought, so to speak. Maybe the mind “grasps” this thing when it becomes an object of apprehension, but again this word lacks the limpidity one could wish for. At any rate, “object” in the intended sense should not be confused with designations of existing objects in space; it has a quite different grammar, as Wittgenstein would say. 
 In this use the word “object” functions like the word “referent”: both are descriptions that allude to a representational medium—mental acts or linguistic expressions. A referent is that which is referred to; an object (of thought) is that which is thought about. It is the same with “denotation” or “designatum” or “subject” (in the sense of “subject matter”).