Molecular biology and systems biology provide answers to the same questions, but the answers are quite different for the two approaches and are, on the surface, unrelated. The molecular biological answers are arguably more fundamental and are unique, but alternative systems biology answers are possible – consider the two systems-level answers to the first question. Nevertheless, the systems-level answers can be in some sense more satisfying, because you feel you have an explanation you can understand and use to make testable predictions; a list of genes together with their interactions is perhaps preferred, but much harder to grasp, explain and remember.That the two types of answer seem to be unrelated reveals an essential flaw in the example I have used. In a really good example, the two levels of explanation would be unified so that the molecular biology explanation would also include the systems biology answers, and vice versa. In our example, the molecular biologist would not only have to identify the genetic networks responsible for cortical patterning, but would also have to discover the molecular basis for the rules that lead to the optimal arrangement of cortical areas and to understand the mechanisms used by the developing cortex to ensure conducting components occupy 3/5 of the volume so that cortical function is optimal. Even if you know all the genetic and epigenetic mechanisms that result in 3/5 of the cortical neuropil volume being conducting components, you still can ask why evolution selected that particular mix of mechanisms. The systems-level approach explains the molecular biologist's answer to this question by appeal to a general principle: evolution selects an optimal partitioning of components.The challenge for systems biology is to discover answers to biology's questions, and the challenge for the molecular biologists is not only to provide answers to these questions, but also to explain the answers given by systems biology. And after molecular biology has given its final answers, the systems biologist must identify the general principles that lead to the actual combination of molecular mechanisms. At its best, then, systems biology and molecular biology work hand-in-hand to provide a complete picture of how and why Biology is the way it is.