Relative price prediction is an inherent part of valuation. You predict what the price of, say, a bond is given the price of the discount rate over the life of that bond. You are not predicting the absolute price of the bond, you are not able to predict if rates are going to go up or down. That's the appeal of arbitrage, you don't need to see the future, you make money no matter what if you see that a particular asset is 'out of whack', mis-priced, cheap, expensive, and you buy/sell it (and execute the appropriate arbitrage hedging strategy until maturity of the trade).