In a face-to-face philosophical discussion, three is a crowd.
If Al 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. This terminological fluidity, though regrettable, is unavoidable since attempt to rigidify terminology will inevitably beg key questions.
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.
Take 'fact.' For some, it is a matter of definition that a fact is a true proposition. But as I use the term, a fact is the truth-maker of a true proposition. Suppose you use 'fact' as interchangeable with
'true proposition.' Then I can accommodate you by distinguishing between facts-that and facts-of. Thus, the fact that Bill is blogging is made true by the fact of Bill's blogging. But we must sort out these definitional questions if we are to make any progress with the substantive issues. A substantive question would be: Are there facts? Obviously, we cannot make any headway with this until we agree on how we are using 'facts.' For more on this topic see Three Senses of 'Facts' and other entries in the Facts category.
And of course we can't stop here. 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? 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.
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.
And so it goes. Suppose Carla is present at Al and Bill's discussion. Will she help or hinder? Experience teaches that, for the most part, three's a crowd: the third interlocutor, in her zeal to contribute to the discussion will only interfere with the protracted preliminary clarification that Al and Bill need before they can get to work on the substantive questions that interest them.
Note 1: The above applies to face-to-face discussions, not to on-line exchanges. Note 2: I seem to recall Roderick Chisholm making the 'three is a crowd' remark. So I may have picked up the thought from him.