necessarily stable: that is, the Phillips relationship could shift. Macroeconomists became more sceptical of Keynesian theories, and Keynesians themselves reconsidered their ideas in search of an explanation for stagflation. The explanation for the shift of the Phillips curve was initially provided by the monetarist economist Milton Friedman, and also by Edmund Phelps. Both argued that when workers and firms begin to expect more inflation, the Phillips curve shifts up (meaning that more inflation occurs at any given level of unemployment). In particular, they suggested that if inflation lasted for several years, workers and firms would start to take it into account during wage negotiations, causing workers' wages and firms' costs to rise more quickly, thus further increasing inflation. While this idea was a severe criticism of early Keynesian theories, it was gradually accepted by most Keynesians, and has been incorporated into New Keynesian economic models. Neo-Keynesianism Neo-Keynesian theory distinguished two distinct kinds of inflation: demand-pull (caused by shifts of the aggregate demand curve) and cost-push (caused by shifts of the aggregate supply curve). Stagflation, in this view, is caused by cost-push inflation. Cost-push inflation occurs when some force or condition increases the costs of production. This could be caused by government policies (such as taxes) or from purely external factors such as a shortage of natural resources or an act of war. Contemporary Keynesian analyses argue that stagflation can be understood by distinguishing factors that affect aggregate demand from those that affect aggregate supply. While monetary and fiscal policy can be used to stabilise the economy in the face of aggregate demand fluctuations, they are not very useful in confronting aggregate supply fluctuations. In particular, an adverse shock to aggregate supply, such as an increase in oil prices, can give rise to stagflation. Supply theory Fundamentals Supply theories are based on the neo-Keynesian cost-push model and attribute stagflation to significant disruptions to the supply side of the supply-demand market equation, such as when there is a sudden real or relative scarcity of key commodities, natural resources, or natural capital needed to produce goods and services. In this view, stagflation is thought to occur when there is an adverse supply shock (for example, a sudden increase in the price of oil or a new tax) that causes a subsequent jump in the "cost" of goods and services (often at the wholesale level). In technical terms, this results in contraction or negative