I've encountered Christians who, in an attempt to argue for a young earth, claim that we could only know if something took place if there were people there to witness it. Since there weren't any people before there were any people, we can't (or at least don't need to) accept the various scientific evidences showing the earth and universe to be billions of years old. The only person there was God, and -- so the young earth proponent argues -- God tells us in the Bible that the earth and universe are young.
Now I have contested the claim that the Bible actually states or implies that the universe is young. Additionally, I have argued that the Bible states that the universe, the cosmos, nature, is an understandable revelation from God to everyone who has ever lived -- that is, people who lived in times and places that had no access to the Bible still received revelation from him through nature, and this revelation was understandable to them without the Bible. In other words, nature does not have to be viewed through the lens of Scripture before we can trust what it seems to say. But in order to take this claim about human witnesses head on, we can more specific: in the Bible the elements of nature are sometimes called upon as witnesses of the events which took place in their presence. For example, God states that heaven and earth will be a witness to his promises to the Hebrews (Deuteronomy 4:26; 30:19; 31:28; Psalm 50:4-6). The prophets call upon nature to bear witness to the truth of their message (Deuteronomy 32:1; Isaiah 1:2; Jeremiah 6:19; 22:29; Micah 6:1-2). Often, stones are set in place or altars are made -- Hebrew altars being simply uncut rocks piled on top of each other (Exodus 20:25; Deuteronomy 27:5-6; Joshua 8:30-31) -- so that these elements of nature can bear witness to promises made between God and people, or just between people (Genesis 28:16-19; 31:43-53; Joshua 22:26-34; 24:26-27; Isaiah 19:19-20). Obviously these latter cases are not exactly the same as the former cases, since they involve human beings altering nature in order to bear witness to something. I'm including them because they alter nature in an extremely limited way, by simply moving a rock into a different position, or moving several into a pile.
The witness of nature (general revelation) is even put side by side with the witness of the Bible (special revelation) (Deuteronomy 30:19/31:19/31:26-28; Psalm 19). This isn't because either of these witnesses can't be trusted by itself -- that we need one of them in order to verify or falsify the other -- but because they are complementary. That is, they are equally valid and true, although not necessarily equally illuminating (obviously, special revelation tells us more about God than general revelation).
This contrasts strongly with the biblical statements about the reliability of human witness. We are warned that we need more than one person as a witness, for the simple reason that people lie (Numbers 35:30; Deuteronomy 17:6; 19:15; Matthew 18:16; 1 Timothy 5:19). A moment's reflection will make evident why there is such a disparity between the witness of nature and the witness of human beings: nature and its elements don't have wills, and thus cannot lie or misrepresent. They can't give a false impression of what has transpired, because they can't alter the effects that events have had upon them. Were this not the case, then all of the Scripture passages which tell us that God reveals his faithfulness and steadfastness through nature would simply be wrong.
So, contrary to this claim that we need human witnesses before we can be justified in believing something took place, the Bible suggests that the witness of nature is more trustworthy. Again, this witness is limited: nature can't tell us about Jesus, it can't tell us about God's salvation plan. But we can't use the fact that it's limited to reject nature's witness altogether. The Bible doesn't allow us to.