Complex sentence: They argue that it is a law of nature that cognitive capacities are productive, systematic and inferentially coherent-they have the ability to produce and understand sentences of a certain structure if they can understand one sentence of that structure.
Split into simpler sentences: They argue that it is a law of nature that cognitive capacities are productive, systematic and inferentially coherent. In other words, that they have the ability to produce and understand sentences of a certain structure if they can understand one sentence of that structure.