Place the potatoes in a large pot of salted water and bring to a boil. Cook until a knife inserted into the flesh meets no resistance, about 25 minutes. Drain. While the potatoes boil, combine the milk, butter, and garlic in a small saucepan and heat until the butter is melted. Keep warm. Use a potato masher or a large wooden spoon to break up the large chunks of potatoes in a rough purée. (If you have a potato ricer, use it—nothing yields smoother mashed potatoes.) Slowly add the hot milk mixture to the potatoes, using a wooden spoon to beat continuously. Season with salt and black pepper.