The human hand has five fingers, and Boston Dynamics has just built a humanoid hand with four. The company unveiled the new hand for its Atlas robot this week, according to Robotics and Automation News, describing it as designed for complex manipulation and tool use while staying strong and durable.

Start with why anyone would remove a finger. Every finger on a robot hand is a set of joints, and every joint needs a motor or a cable, plus a way to sense where it is. More fingers means more parts that can jam, snap or wear out. A hand that swings a tool and takes knocks needs to be tough, and the plainest way to make it tougher is to have fewer things inside it that can fail.

The catch is that fewer fingers can mean less skill. Holding a screwdriver or a wrench means pressing on it from several sides at once, and each finger you lose is one fewer contact point. So the engineering question is how few fingers still hold a tool firmly and still let the robot reorient it. Boston Dynamics is betting that four is enough. The announcement says what the hand is for. It does not, in the material available to me, say how well it does it.

That gap defines the stage. This is an unveiling of a design for a humanoid, with no durability figure, no grip-strength figure and no deployed-hours figure in the coverage I can see. A hand that looks right in a demo has not yet been through weeks of repeated grasping, which is where robot hands actually fail. The company is selling a promise of both dexterity and toughness; the company has shown the shape of the hand, and the second half of the promise is still unproven in public.

The next gate is dull and decisive: footage of Atlas using ordinary tools on a task nobody rehearsed, followed by a number for how long the hand runs before someone has to open it up and fix it. Until then, the missing fifth finger is a hypothesis about how few fingers a working robot needs.