ConLing - What constitutes a philosophical language?

It's been rare that I cover a topic that's exclusively conlang related, but I think this qualifies. Philosophical languages certainly don't occur in the wild, and even the most successful ones have never picked up fluent communities the way some lucky constructed languages of other types have. Indeed, the matter of whether or not a philosophical language can even be learned and used in the same way as a natural language is subject to debate - by definition they entail some major departures from the way humans naturally communicate. But what exactly are they?

A sample of Wilkins' Real Character

It's difficult to define the exact boundaries of what does and doesn't qualify as a philosophical language - the definition has something to do with the creator's intention as well as the actual structure of the language. Generally, a philosophical language is one engineered with major structural differences from natural languages, which are meant to reflect the creator's philosophy of how communication and the synthesis of meaning could or should work. They may or may not, though often do, come with implicit assertions that the language has the power to improve or bring clarity to the human condition. Much to the chagrin of many a philosophical language maker, the very things that make them special also tend to irreparably impair their potential as practical communication tools for real people. What do I mean? As always, a few examples might help paint a picture.