Being an engineer, my answer is skewed to the engineering side. Engineering teaches us discipline and process, without which project management cannot be successful. I see humanities project managers who downplay the value of discipline and process and wonder why they struggle with their projects (or use process to make up for a lack of discipline). But engineers need to recognize the value of humanities to be truly successful project management professionals. Humanities are needed to add perspective. Some universities are starting to offer degrees that cross disciplines. A subject like history teaches us the value of lessons learned (or perhaps the mistake of not learning our lessons) and good governance, languages for communication, the arts for creativity, critical evaluation, identifying details, philosophy or theology for deeper understanding of cultures, empathy, value systems. The beauty of project management is that the environment pushes us to be constantly learning. It is not formulaic, especially in our global international marketplace. Our profession and standards like PMBOK, PRINCE2, ISO 21500, DIN, etc. provide us with the building blocks, the humanities provide us with the perspective to build the 3 dimensional pyramid.