The Fact of Terminological Fluidity
If Peter and Bill are talking philosophy, the first thing that has to occur, if there is is to be any forward movement, is that the interlocutors must pin each other down terminology-wise. Each has to come to understand how the other is using his terms. It is notorious that key philosophical terms are used in different ways by different philosophers.
The following is a partial list of terms used in different ways by different philosophers: abstract, concrete, object, subject, fact, proposition, world, predicate, property, substance, event.
The same holds no matter what the subject matter, and understanding this is a mark of an educated person. There is no point in debating whether Obama is a socialist until one has defined or at least explained 'socialist.' Would that the gasbags of contemporary punditry appreciated this simple point!
Returning to 'fact,' if you say that a fact is a true proposition, then I will ask you how you are using 'proposition.' Do you mean the sense of a context-free declarative sentence? Are propositions for you abstract objects? Are you Fregean when it comes to propositions? But now we need to get clear about 'abstract' and 'object.' Do you use 'object' and 'entity' interchangeably? Or can there be objects that are not entities and entities that are not objects? (An hallucinated pink rat might count as an object that is not an entity, and a being that has never been the accusative of any intellect might count as an entity that is not an object.) Someone who uses 'object' in such a way that there is no object without a (thinking) subject is not misusing the word: that is a traditional use. But equally, a person who uses 'object' to mean entity is not misusing it either. So the use of 'object' needs clarification.
'Object' is used in at least three ways, none of them wrong: (i) object = entity; (ii) object = intentional object = accusative of a mental act; (iii) object = particular. I just used 'particular.' But what is a particular? I would explain 'particular' by contrast to 'universal' and say that a particular is any item that is unrepeatable, while a universal is any item that is repeatable.
I just used the word 'item.' What do I mean by that? As I use it, 'item' is the most noncommittal of terms, the ultimate in terminological neutrality. It does not beg any questions or import any theories. An entity (from L. entitas, from ens, being) is anything that exists or has being, with 'existence' and 'being' being used interchangeably. 'Item,' however, is broader than 'entity' since an item may or may not exist. An item is anything one can think about or refer to in any way. Suppose a Meinongian distinguishes among existence, being, and Aussersein (extrabeing). Then 'item' is neutral with respect to that three-fold distinction. 'Item' commits me to nothing except self-identity. Cf. Latin idem. An item is an item. No item is self-diverse. Self-diversity is of course an item, but it is not self-diverse! Why is self-diversity an item? Because I just referred to it. Nothing is self-diverse, and nothing can be self-diverse, so this 'property' cannot be a property of anything. Yet we are now thinking about it and saying true things about it: self-diversity is not self-diverse; it is what it is; it is not possibly instantiated; it is diverse from self-identity; etc.
But what about the Meinongian item, the self-diverse item? Is it self-diverse or not? This looks to be a puzzle for a Meinongian, but since I am not a Meinongian I don't have to solve it.
Returning to 'abstract' and its opposite, one might use 'abstract' and 'concrete' as follows: X is abstract (concrete) iff X is causally inert (causally active/passive). But I know of at least one name philosopher who uses 'abstract' interchangeably with 'nonspatiotemporal.' On this usage, God would be an abstract object, while on the first definition God would be concrete.
Note that an abstract entity on either of these two definitions can be a substance (another word with about ten meanings!), i.e., a being capable of independent existence. But 'abstract' is used by philosophers as diverse as Hegel and Keith Campbell (the Aussie trope theorist) to refer to non-independent objects. And indeed, their use is the classical, and etymologically correct, use.
I have merely scratched the surface of terminological fluidity in philosophy. I could write pages more (and you hope I won't). The main point is that, before 'cutting loose' in a discussion one must carefully define one's terms and reflect on how they cohere with one another.
The Unavoidability of Terminological Fluidity
The fact of terminological fluidity is undeniable. It is unfortunate in that it impedes understanding and communication. Time is consumed by mere terminological clarification. But nothing can or should be done about it. For the terminology we craft, and the distinctions we draw, may limit or enable or interfere with the claims we advance. Suppose there were a Central Terminology Authority (CTA) and it ruled that 'exist' shall mean: to be instantiated. It would follow from this terminological fiat that no individual can exist. For it is obvious that no individual can be instantiated: 'Socrates has instances' is nonsense. And yet it makes sense to speak of the existence of individuals. Or if the CTA were to rule that 'cause; is to be defined as a relation between events, this would rule out agent causation, which is the classical notion of causation.
Terminological fluidity must be tolerated in a discipline as open-ended as philosophy. Kierkegaard compares philosophizing without dogma to sewing without a knot at the end of one’s thread. His thought is that the reasoning process must be anchored in premises whose truth is beyond question; otherwise, one will never arrive at a fixed conclusion. This invites the rebuttal that to philosophize with dogma is not to philosophize at all. Philosophy is radical reflection: it cannot take anything for granted, as immune from examination. This is the very idea of philosophy. If so, then a fixed and unquestionable terminology is as anathema to the true philosopher as unquestionable premises.