And additionally to complete your statement is to have stable capital. Although patience and strong mentality are factors that are the key to success in trading. Stable capital is also needed as a complement to the two categories that have been mentioned, especially for beginners who are still in the process of gaining knowledge so with have stable capital, patience and strong mentality they can build a strong foundation to achieve long term success.
Stable "income" would make more sense. If you have stable capital, that means you are going to just have the same money, sure you are not withdrawing, but you are not putting in more. But if you can have a better job then you are going to have more income and that is better, most people do not even do that and they do not put any from their income at all.
Just make sure that you are dealing with something that makes you more money each month and invest some of that every month. That will allow you to get greater results. If you have just one capital and try to grow that, it will be slow and not much, but if you put in more money and accumulate more all the time then it will allow you to get result you would prefer more.