We Don’t Have to Know What It Is

In a 2011 talk on Isaac Newton and philosophy of mind given at the University of Oslo, Noam Chomsky observed, “There hasn’t been any concept of ‘physical’ for hundreds of years. [...] The physical these days are things that Newton would have regarded as total absurdities.” This essay illustrates the historical and metaphysical contingency of our concept of the physical by comparing 19th-century work on energy with the natural philosophy of earlier centuries as well as with modern physics. Section 1 briefly summarizes the historical context surrounding James Joule’s 1847 argument for the mutual convertibility of natural powers, paying special attention to its metaphysical motivations and challenges. Section 2 contrasts the concept of the physical developed by Joule and his contemporaries with earlier, Cartesian and Newtonian notions of mechanical physics. Conversely, section 3 contrasts Joule’s energy-conservative concept of physics with our modern notion of ‘physical’ spacetime. Finally, section 4 considers the implications of this history for contemporary philosophy of science: Over time, physics has become more predictively powerful at the same time as it has become less tangibly ‘physical’.