Economics is a science, like weather forecasting is a science. There are so many shifting variables, including human behavior, so our scientific inquiry has limits. Perhaps it is fairer to say that economics is part science, part art?
More specifically, economics is a social science. That is to say, it is a science of human behavior. Human behavior isn't a variable in economics. That would be like saying mathematics is part of algebra. In reality, economics is a part of human behavior, just as algebra, calculus, or trigonometry, is a specific study within the field of mathematics. Economics is, at its essence, an effort to understand and explain human behavior--behavior related to the exchange of goods and services.