Internal critique is the method whereby a particular position is examined upon the basis of its own presuppositions. Presuppositions are the most basic tenets of a particular position or worldview. Assuming an internal critique is positively successful, the position in question will espouse tenets fully consistent with its presuppositions. Insofar as the critique is negatively successful, the position in question is found wanting due to inconsistencies, arbitrariness, contradictions, and the like.
Rather than examining a position upon the basis of its own presuppositions, external critique examines said position in virtue of the presuppositions or derivative tenets of another position or worldview external to the original position in question. An external critique is positively successful insofar as the two positions in question allow for syncretism. A negatively successful external critique finds the initial position to be in error because it does not comport with whatever other position is used as the basis for critique.
Negative External Critique
By honoring Christ as Lord and taking every thought captive, the consistent Christian is always engaging in external critique, whether he or she gives explicit expression to said critique in an apologetic encounter or not. There are positions that the Christian may encounter and reject merely because the Christian worldview does not allow for them to be true. In such cases the Christian may not have an internal critique readily available, but rationally rejects the position upon the basis of the frank acceptance of the Christian faith.
When an apologist gives expression to a negative external critique he or she proclaims the utter folly of unbelieving thought from within the confines of the Christian worldview. This can be every bit as persuasive as internal critique. It is very similar to preaching, and may involve reasoned presentation of the consistency inherent to Christianity, though it need not use argument in the aforementioned intellectual, rationalistic sense.
Every successful internal critique depends upon external critique. Any position which is rendered unlivable, incoherent, arbitrary, absurd, and the like provides no inherent basis upon which it may be recognized as such. Even the aforementioned categories utilized in internal critique are contingent upon the position that takes them to be necessary features of internal critique. Internal critique is thereby established, rather than rendered obsolete, due to its usefulness as a means of expressing the truth of the Christian faith.
Objections to the validity of external critique as it may be understood within the Christian worldview themselves stem from inconsistent dependence upon external critique. The transcendental challenge is thus deep indeed, penetrating every position down to its presuppositions, and bringing them into play at every level of the game.