Keynes acknowledged the historical significance of the labour theory of value as a foundation laid by classical economists. However, he was critical of its ability to explain the complexities of modern capitalist economies. Specifically, Keynes argued that:

1. Value Determination Is Multifaceted:
    Keynes maintained that relying solely on labour input oversimplified the process of price formation. For him, market demand, consumer preferences, and uncertainty play a definitive role in setting prices, which cannot be fully explained by aggregating labour inputs.

2. Role of Capital and Uncertainty:
    Keynes underscored that modern economies incorporate significant capital investment and speculative elements not readily captured by a pure labour measure. The presence of fluctuating returns on investment, risk, and the marginal efficiency of capital suggests that the market value of commodities is determined by a more complex interplay of factors than the classical labour approach.

3. Practical Challenges in Measurement:
    He also pointed out the practical difficulty in measuring the ‘socially necessary’ amount of labour that goes into the production of a commodity, noting that technological advancements and changing production processes complicate such assessments. This further diminishes the explanatory power of the labour theory when applied to contemporary economies.

In essence, while Keynes recognized the importance of labour as a component in production, he argued that a complete understanding of value requires consideration of marginal factors, market dynamics, and the uncertain nature of economic decision-making. This broader perspective moves beyond the deterministic framework of the labour theory of value and aligns more closely with his own theories on effective demand and the behavior of modern markets.