Of course you can do more than one thing at once. The catch is you can't do more more than one thing at once that requires concentration. You can walk and chew gum, but you can't write an email and a report at the same time.
When you're doing two things at once that are cognitively demanding, what you're really doing is switching back and forth very quickly from one task to another.
The brain is a pretty smooth operator when it knows what direction to head. Transitioning from one task to another, however, dramatically reduces its horsepower.
There is a mental cost—called a switching cost—to all of this multitasking. But that's not all it makes deep thinking impossible.
Steven Yantis, a professor of psychological and brain sciences at Johns Hopkins, says:
In addition to the switch cost, each time you switch away from a task and back again, you have to recall where you were in that task, what you were thinking about. If the tasks are complex, you may well forget some aspect of what you were thinking about before you switched away, which may require you to revisit some aspect of the task you had already solved (for example, you may have to re-read the last paragraph you’d been reading). Deep thinking about a complex topic can become nearly impossible.
Multitasking ensures the brain is operating sub-optimally.
When you try to do two or more cognitively demanding things at once, it's as if you have a 1000 h.p. motor (your brain) but you're throttling is so you only get 100 h.p. of output.
There is a difference between the appearance of being busy—busywork—and actually moving the needle. Attempting to do more things may signal to others how busy you are, but if you care about actual results, you need to do fewer things better.
Our most valuable mental habits—things like deep and focused thought—must be learned through concentrated practice. This is a skill we're starting to lose as more and more of our time gets fragmented away into small and smaller increments thanks to screens. Nicolas Carr, author of The Shallows: What the Internet Is Doing To Our Brain, comments on his blog:
Our most valuable mental habits – the habits of deep and focused thought – must be learned, and the way we learn them is by practicing them, regularly and attentively. And that's what our continuously connected, constantly distracted lives are stealing from us: the encouragement and the opportunity to practice reflection, introspection, and other contemplative modes of thought. Even formal research is increasingly taking the form of “power browsing,” according to a 2008 University College London study, rather than attentive and thorough study.
Patricia Greenfield, a professor of developmental psychology at UCLA, warns that our growing use of screens appears to weaken our “higher-order cognitive processes,” including “abstract vocabulary, mindfulness, reflection, inductive problem solving, critical thinking, and imagination.”
Busywork leads to overwork and sub-optimal outputs. Busywork is a psychological signal to others (and ourselves) that we're needed. That we're important. After-all, look how busy I am. This delusion is the reason we take on so many things and fail to say no. This delusion keeps us so busy that we're not learning new things or even getting better at what we already do. This delusion inhibits us from deriving meaning in our work and our lives. This delusion keeps us from understanding how the world works and adapting to the changing reality.
The world is a competitive place. That means we have to learn how to do difficult things — things that other people value, things other people can’t do. This isn’t as easy as it sounds. What we really need to develop is not so much the specific traits that will be valued as much as a system to constantly reinvent ourselves and adapt.
This is the hard part and in an odd way, we need to go backwards to go forward. We need to develop skills we've once had and lost: The ability to read better, the ability to make better decisions, the ability to focus, and strategies to make sure we're not consumed by busywork. These are the skills that will enable us to have quality ideas that other people don’t have. These are the foundational skills that underpin adaptability.