I hear this phrase misused often: "It's an art, not a science."
To me, this phrase really means that the way something is judged is not objective; rather, it is dependent on the beholder, and different pieces speak to different people in different ways. Jackson Pollock, the artist, had a very distinct, but controversial, style. Some people liked it, and some people didn't. It couldn't be judged objectively, because it's art, not science.
You could apply science to this art, by the way. You could look at color density, color distribution, scope, size, effort. And all those things that you could measure about it could not predict what any specific person's reaction to it should be. It's that lack of predictive ability that prevents art from being science.
I heard this phrase, "It's an art, not a science," used a couple days ago to describe project management. I've heard it applied to creating organizational structures, or defining a management process. In all these cases, this phrase is being used as a placeholder. An excuse. A conversational roadblock.
Just because you don't want to codify your decisions does not make what you're doing art. It just means you're being sloppy and don't want to talk about it anymore. Spend the time. Convince your peers that what you're doing is the right thing at the time. If it's a one-off, you still have reasons. It's not art; it's a result of a calculated business decision.
Just because something is difficult to do doesn't make it an art. It just means you have to work harder and adjust more to get it right. Take the time to create metrics and repeatable processes. When you have repeated things a couple times, then tweak and look for different results. You can't do experiments in an uncontrolled environment.
Just because you can't articulate why you have a gut feeling doesn't make it an art. It just means you're not communicating clearly, and if you want people to listen to you and get behind you, keep clarifying. People who follow "gut" leaders are not the people you want executing for you, because they're not questioning deeply enough.
Just because you can't manage to do things the same way twice doesn't make it an art. It just means that you lack the discipline and will to do things the same way twice. You failed to collect the requirements completely on this project? Doesn't mean that it's optional. It means you failed on that part and that you need to push consistency. It doesn't make you an artist to fail to follow your defined process or follow through on your commitments.
Project management, creating organizational structures that work, creating cohesive teams, and creating management processes are all disciplines. They each have a body of literature about how to do things in an optimal way. There are objective ways to define metrics, make measurements, check against metrics, tweak and optimize. There is very much a scientific approach to doing all these things. Anyone who says otherwise isn't reading enough about their own discipline.
Beware of anyone that says "It's an art, not a science." Call them on it. Often, they're trying to end the conversation because they don't have a good argument. Because what they want isn't really the scientific optimization, but some other, more politically motivated answer. In the interest of openness and transparency, call them on it. Bring the underlying issues to light. Help guide them to the principles that will help your organization grow strong with evidence-based decision making.