However, in the Fourth Meditation , Descartes goes to great lengths to guarantee the truth of whatever is clearly and distinctly understood. This veridical guarantee is based on the theses that God exists and that he cannot be a deceiver. These arguments, though very interesting, are numerous and complex, and so they will not be discussed here. Suffice it to say that since Descartes believes he has established God’s inability to deceive with absolute, geometrical certainty, he would have to consider anything contradicting this conclusion to be false. Moreover, Descartes claims that he cannot help but believe clear and distinct ideas to be true. However, if God put a clear and distinct idea in him that was false, then he could not help but believe a falsehood to be true and, to make matters worse, he would never be able to discover the mistake. Since God would be the author of this false clear and distinct idea, he would be the source of the error and would, therefore, be a deceiver, which must be false. Hence, all clear and distinct ideas must be true, because it is impossible for them to be false given God’s non-deceiving nature.