Christian philosopher Michael Butler affirms in his article that conceptual necessity does not guarantee ontological necessity. In other words, we basically need more than a "conceptual scheme" to provide knowledge; we need an "actual" (ontological) scheme as well. Reformed apologists espousing the Transcendental Argument for God's existence (TAG) employ the idea that the Christian God as well as his revelation, the Bible, are the preconditions necessary for knowledge.
TAG tries to prove Christianity indirectly by the "impossibility of the contrary" by showing the absurdity of the non-christian worldview (religion, or or theory of life), and showing how only the Christian worldview can make sense of science, logic, and morality, and indeed--all of life. Too much is to be written to defend TAG at this juncture, for the criticisms abound against this argument both within the Christian world and without it (the "classical" arguments for God's existence also have this 'back-and forth' in common with TAG i.e. the cosmological, teleological, and ontological arguments).
Often proponents of TAG show how it is conceptually necessary to have a personal, triune God who controls the universe, gives uniformity to nature (metaphysics), holds comprehensive knowledge and shares portions of it (epistemology) and serves as moral law-giver, standard and judge of morality (ethics); where they sometimes fail is to show how this concept is ontologically necessary. In other words, the unbeliever may finally 'cry uncle' and admit his worldview does not support absolutes, because he espouses a relativistic concept of morality, and he may admit that morals only make sense if there is a personal God (an actual Person) who gives moral commands to humans and serves as a transcendent, ultimate authority and standard. So, the concept is the only one acceptable, the unbeliever admits. However, does this mean the Christian has convinced the unbeliever of the ontological necessity of of Christianity?
This is why Butler and others (Bahnsen, Van Til, Frame, Schaeffer--sort of) in the reformed apologetic school of presuppositionalism assert that not only is revelation from God necessary as a concept but also necessary ontologically. This ontology, this "actuality" is found in the Bible itself. Butler, in his article is trying to refute the idea that the Christian apologist has done his service by only showing the concept of Christian theism as being necessary for knowledge, morals, and science. Rather, said apologist must assert the Bible--as special revelation--as the "proof in the pudding," the ontological answer to our queries concerning the aforementioned fields in philosophy.
I was thinking about this needed congruence between conceptual and ontological necessity the other day while digging up rocks in the front yard because the landscaper didn't give us enough(any?) topsoil when he planted the grass. I remember my physics teacher in high school teaching that for sound to occur, there have to be three aspects present: source, medium, receiver. This made me think of the Trinity, as it has a lot to do with TAG in the area of epistemology, metaphysics, and ethics. Such natural triads, in this case, concerning the necessary preconditions for noise to occur, are found throughout our natural world: water is solid, liquid or gas; education has the trivium (grammar, dialectic, rhetoric); the human body is broken down to bone, tissue, liquid. (There are many other triads listed in John Frame's Doctrine of God).... So, I wondered about the significance between the necessity of the three requirements for sound as it relates to the three persons of the Trinity. In the redemptive work of God, the Father initiates, the Son accomplishes, and the Holy Spirit applies salvation to people.
But does that mean that sound, or thought required the ontological Trinity? If there were only two persons in the Trinity, eternal counsel could still be had because there would be a source and receiver, while the medium would be space. But, if God is the creator of everything, then he is the creator of space as well. It could be argued that space did not exist as a medium until it was actually created. Furthermore, in the Christian concept of Trinity, God is three persons in one God, not two separate gods. The concept of the Trinity as three-in-one lays the perfect foundation for not only unity and diversity in the community of the Trinity (Ravi Zacharias), but also as a paradigm for us to view the natural realm, in this case, that of the necessary elements required for sound to occur. In the Trinity and the eternal counsel of God, their is indeed a source, medium and receiver. However, the "lack of space" argument may not work, as God is a spirit, and requires not space. If it is argued that God does not require space, then a two-person Godhead may suffice to explain discourse. Even so, I think Frame is right by saying the triads in the natural realm do not prove the Trinity as ontologically necessary, but rather reflect the glory and design of the Triune God. The trick here is that this type of information usually only 'clicks' with people who already affirm belief in the Trinity. I don't know if triads in the natural realm show the preconditions of the ontological Trinity as necessary to make sense out of human experience as much as they reveal what God has done. And, not everyone believes the Christian God has done anything. But, I was just picking rocks when I thought of this, so I'll have to hash it out some more...
Nevertheless, the idea that a concept can be required without an ontology to go with it is repudiated by Butler. This is most likely because, not only such an idea fails to prove TAG with any emphatic scope, but rather because it is possible that concepts in and of themselves may not exist apart from an ontological reality. A similar problem may be the "mind/body problem," but the real objection here is that merely proving Christianity as the precondition for the intelligibility of human experience as a concept does not force the unbeliever to accept Christianity ontologically--as the actual truth in existential terms.