Final answer:
The original Phillips curve's weakness was its failure to consider the shifting relationship between inflation and unemployment, as observed during the period of stagflation in the 1960s.
Step-by-step explanation:
The weakness of the original Phillips curve is that it ignored the possibility of shifts in the curve itself, meaning that it did not account for the fact that the relationship between unemployment and inflation could change over time. Initially, the Phillips curve suggested a stable, inverse relationship between the rate of unemployment and the rate of inflation, implying that with higher unemployment, there would be lower inflation, and vice versa. A.W. Phillips analyzed data and established this concept, but it eventually became clear that this relationship was not as stable as once thought, particularly during the 1960s, when the phenomenon of stagflation (the coexistence of high inflation and high unemployment) observed in the United States challenged the original Phillips curve concept. This led to an understanding that the curve could shift due to factors such as inflation expectations, thus disrupting the simple trade-off between inflation and unemployment.