The test vector problem and limitations to evolving digital circuits

How do we know the correctness of an evolved circuit? While Evolutionary Hardware is exhibiting its effectiveness, we argue that it is very difficult to design a large-scale digital circuit by conventional evolutionary techniques alone, if we are using a subset of the entire truth table for fitness evaluation. The test vector generation problem for testing VLSI (Very Large Scale Integration) suggests that there is no efficient way to determine a training set which assures full correctness of an evolved circuit.