The central question of this dissertation is how much of judicial decision-making depends on legal reasoning. Do judges, after finding the relevant facts of the case, consult legal rules and then arrive at their decision? Or maybe the equation that the decision equals facts plus rules is merely an illusion? This dissertation argues that judges usually can make decisions on other grounds than formal legal rules and then use formal legal rules merely to justify those decisions. Also, judges will have preference for intuitive decision-making over rule-based and logical reasoning. When judges make decisions on other grounds than formal legal rules, judicial creativity is unlikely to be constrained by these formal rules. Also, judges will seldom have trouble justifying their decisions with formal rules because they will almost always find some competing legal rules that will tally their decision; this is largely because public international law is even more ambiguous than common law systems. Specific driving forces behind judicial decisions will be different in each case – it is even possible that in some cases formal legal rules will be the controlling factor. However, international courts will be often swayed by various policy principles, including conciliatory justice. It also means that judicial law-making by international courts is inevitable.