Compared to an experienced trader, is he or she can be 100% sure which direction market will go? Definitely no.. there is no such thing as holy-grail. Therefore, professional trader will never risk more than he/she can afford to lose. However, if this is true..then there will never have ppl making huge profit from the market. So obviously, those ppl who made big money from trading is some sort of gambler, ALWAYS. Since not even professional trader can be 100% sure about the market direction, this is where martingale comes to its edge. Especially in short term trading, which is purely speculation, you only need to guess correct market direction sometimes(not everytime/every analysis) over the long period, you can make money with martingale.