My thesis, as the title refers, is about practical reasons and rationality. The purpose of this thesis is not to formulate any final theory on what practical reasons are or what it means to be rational, but rather to present and discuss Bernard Williams’s “internalism of practical reasons” and argue that an externalist alternative is more plausible. In his famous and influential paper “Internal and External Reasons” Williams discusses what it means that “A has a reason to ö” or “there is a reason for A to ö”. “A” refers to an agent and “ö” refers to any verb of action. Williams argues the “internal interpretation” to be the most preferable and he presents several arguments against the “external interpretation”. Williams’s internalism of practical reasons claims that all practical reasons are based on, or provided by, what he calls our “motivational set”. This means, roughly, that it is not true that “A has a reason to ö”, unless he is motivated to ö or in other words, A has a current desire to ö. Externalism of practical reasons claims that there might be true that there is a good reason for A to ö, independently of A being motivated to ö. According to this view, our practical reasons are based on value. It is what may be relevantly good about an action that motivates us to perform this very action. I am presenting and defending this view in my thesis and it is from this externalist position I am discussing Williams’s internalism. Following Derek Parfit I am arguing that no reasons are provided by our desires. Good reasons for acting are based on value, and we are rational insofar as we respond to our good value-based reasons. Most of my arguments are either influenced by, or based on, the writings of John McDowell, Derek Parfit, Warren Quinn, Joseph Raz and T.M. Scanlon. I am arguing that we do have reasons to prefer the externalist position rather than Williams’s internalist position and that Williams’s arguments against externalism are no threat for the externalist position I am presenting in my thesis. It is possible that there might be a good external reason for an agent to ö, independently of this person’s current motivations. This entails that not all reasons are provided by our motivational states. If this is true, as I am arguing that it is, then Williams’s arguments against externalism fails. Furthermore, I am arguing that no reasons are provided by our motivational states. If this latter bolder claim is true, then internalism of practical reasons is false. Or so am I arguing.