Bahnsen's book on Van Til does a good job of summing up why and how non-Christian philosophy has not been able to harmonize universals with particulars.

At any rate, I feel that the presuppositionalist argument is very strong - the notion that, absent God, the notion of "proof" itself would be impossible. That only the existence of God can explain logic, numbers, minds, universals, particulars, infinites, etc. But I think the presuppositionalist needs to be ready to study their philosophy in depth - some have been guilty on relying on snappy answers without much depth. So if the unbeliever asks where is the proof that God is the source of logic, abstracts, etc., then you should have an answer ready.

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Vaughn Shideler
St. Andrew's Presbyterian Church
Toronto, ON, Canada