Non-Christians can and do engage in activities using logic, science, and morality. Christians do as well. Presuppositionalists claim that these two groups can do so only because the world is what God says it is.
The argument advanced for this claim begins with one of the accepted activities mentioned above (logic, science, or morality) and illustrates how this activity is possible if the world is what God says it is. Then the accepted activity is shown to be inconsistent with what anyone else other than God says the world is.
While it might be said that the non-Christian cannot and does not engage in these activities consistent with his or her worldview, it cannot be said that the non-Christian cannot or does not engage in these activities. The activities are carried out in spite of the unbelieving worldview; being borrowed from the Christian worldview.
This claim may appear to be rather simple but it gets missed, misunderstood, or misrepresented a lot. To paraphrase Van Til, “Unbelievers can count, but they cannot account for counting”.