To see how this might work in the brain, begin with a highly simplified view of elements such as hypotheses and evidence as represented by single neuronlike units rather than by patterns of activation in neural populations. We can then build an artificial neural network that represents constraints among elements by links between the units that stand for them. Figure 4.2 shows a simple network that has units representing competing hypotheses in the Simpson case. Positive constraints based on what explains what are captured by excitatory links between units, roughly analogous to the synaptic connections that enable one neuron to excite another. Note that figure 4.2 allows levels of explanatory hypotheses, with the hypothesis that Simpson was angry at his ex-wife explaining why he killed her, which explains why she is dead. Negative constraints are captured by inhibitory links between units. Another positive constraint that affects the network is that we should tend to accept what we have observed with our senses, in this case that Nicole is dead.