A popular metaphor for the representation of numbers in the brain is the 'mental number line', in which numbers are represented in a continuous, quantity-based analogical format1,2. Here we show that patients with hemispatial neglect3 misplace the midpoint of a numerical interval when asked to bisect it (for example, stating that five is halfway between two and six), with an error pattern that closely resembles the bisection of physical lines4. This new form of representational neglect constitutes strong evidence that the mental number line is more than simply a metaphor, and that its spatial nature renders it functionally isomorphic to physical lines.