A common objection or typical misrepresentation of what Van Til taught in respect to presuppositional apologetics is that nonbelievers cannot or do not know anything. This usually stems from a misunderstanding of how the Christian view of epistemology works in practice and in principle regarding the nonbeliever as espoused by people who adhere to presuppositionalism. But first, let us allow Dr. Van Til speak for himself on this point as this objection will often be raised in the form “I heard Van Til wrote that nonbelievers don’t know anything.”
Still further, it is when we presuppose with Calvin that all men inherently know the truth, because they and the universe about them are made by God, and then we assert with Calvin that all men are spiritually at enmity against God so that they are anxious always and everywhere to suppress the truth, that we can also speak with Calvin of God’s common grace by which men are able to cooperate with believers in building the structures of science. As far as natural ability is concerned the lost can and do know the truth and could contribute to the structure of science…
It is thus in the mixed situation that results because of the factors mentioned, (1) that every man knows God naturally, (2) that every sinner is in principle anxiously striving to efface that knowledge of God, and (3) that every sinner is in this world still the object of the striving of the Spirit calling him back to God, that cooperation between believers and unbelievers is possible. Men on both sides can, by virtue of the gifts of God that they enjoy, contribute to science. 
It is quite clear that Van Til taught that through God’s common grace (John 1:9, Matt 5:45) to all mankind, and their creatureliness of being made in the image of God (Gen 1:27, Rom 1:19-22), that nonbelievers are able to know things in practice even though their principles do not give them the necessary preconditions for such knowledge to be obtained. Nonbelievers are able to know things, but only by their inconsistency in what they believe and only by the grace of the God they deny.
This is a good example of how Van Til was applying these and other doctrines of historic Christianity at the level of apologetics and doing his best to be consistent in all areas of thinking and application in order to have a Biblical apologetic.
Others have addressed this as well, namely Dr. James Anderson and Chris Bolt with quotes of Van Til in his various works.
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