We argue that there is a tension between two types of design arguments-the fine-tuning argument (FTA) and the biological design argument (BDA). The tension arises because the strength of each argument is inversely proportional to the value of a certain currently unknown probability. Since the value of that probability is currently unknown, we investigate the properties of the FTA and BDA on different hypothetical values of this probability. If our central claim is correct this suggests three results: 1. It is (...) not very plausible that a cumulative case for theism include both the FTA and the BDA (with one possible qualification); 2. Self-organization scenarios do not threaten theism but in fact provide the materials for a good FTA. 3. A plausible design argument of one sort or another (either FTA or BDA) will be available for a wide variety of values of the key probability. (shrink)