Sorry if this is a bit lengthy and technical, but this is actually more like a summary of what Says Law is trully about:

Reality is characterized by scarcity, while humans have unlimited desire/demand. Humanity use resources from nature to produce goods and services that others desire/have demand for. Everyone are both producers and consumers. For people to get goods or services they must provide give something in exchange. This is usually money – the medium of exchange. Therefore people must usually be productive if we want to consume goods or services, so that we have capability to provide something in exchange for the goods or services we desire. For people, money is not an end in itself but a means to an end. Money is a means by which people consume in accordance with their desires. Individuals must produce things desirable to others. Therefore it is others’ demand for goods or services one may produce which induces a producer to produce a good or service, not labour-time. The necessity for economic actors to achieve profit and avoid loss incentivizes producers to focus on economic activity which is most productive and thus, most profitable. Consequently, in a properly functioning economy, production is automatically directed towards producing supply of goods and services for which there is demand. Therefore without state intervention being necessary, the market naturally allocates resources and money towards production most conducive to both profit, and the satisfaction of consumer demand. All economically active people, i.e. most people. Are therefore simultaneously producing goods/services and consuming goods/services in the economy. This occurs voluntarily and is emergent from many individuals with independent desires, all acting independently, and from this harmonious economic prosperity emerges naturally without state intervention. Therefore it is necessary for there to a be a free market with minimal state intervention, to ensure this process operates properly, for if this process is interfered with, the allocation of resources in the economy towards profitable and productive activity does not occur efficiently, and mistakes are made – misallocations of resources. Misallocations of resources which ultimately lead to economic crisis. Economic crisis is therefore caused by state intervention, rather than being the inevitable result of market activity. So basically, Marx was wrong, Keynes was wrong, the market is neither inevitably doomed to failure, nor is there a necessity for the economy to be interfered with to operate optimally.