Page "Risk-neutral measure" Paragraph 4

The lack of arbitrage is crucial for existence of a risk-neutral measure.

In fact, by the fundamental theorem of asset pricing, the condition of no-arbitrage is equivalent to the existence of a risk-neutral measure.

Completeness of the market is also important because in an incomplete market there are a multitude of possible prices for an asset corresponding to different risk-neutral measures.

It is usual to argue that market efficiency implies that there is only one price ( the " law of one price "); the correct risk-neutral measure to price with must be selected using economic, rather than purely mathematical, arguments.