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  1. Robots are too complex to be built.
  2. Robots are too complex to build.

I'd like to know which is correct.
Thanks a lot.

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They mean much the same.

Perhaps some light on any nuanced differences can be shed by adding an agent phrase and then examining the prepositions we use with each.

A microbe is too small for the naked eye {to see}.

A microbe is too small {to be seen} by the naked eye.

The car is too expensive for the average wage-earner {to buy}.

The car is too expensive {to be bought} by the average wage-earner.

Semantically, we supply [the implied object] with the first (if it is not present, though it can be):

A microbe is too small for the naked eye {to see} [it].
The car is too expensive for the average wage-earner {to buy} [it].

but that step is not necessary with the second, the by form.

The idea is

the naked eye cannot see the microbe

the average wage-earner cannot afford the car

Is this idea expressed with any greater directness or emphasis by either of the versions?

the naked-eye cannot see it
it cannot be seen by the naked eye

the average wage-earner cannot afford it
it cannot be afforded by the average wage-earner

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Both sentences are correct and have essentially the same meaning.

However the first sentence

Robots are too complex to be built.

Has greater emphasis on being not buildable

Robots are too complex to be built (ever, by anyone).

Robots are too complex to build.

Could also mean the same as to be built, but can also mean

by someone at this present time given the current circumstances

Robots are too complex (for me) to build
Robots are too complex to build (on our budget)

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  1. The active infinitive is prefered if you're thinking more about the person who will do the action.
  2. The passive infinitive is used if you are thinking more about the action, or the person/thing that the action is done to.

    • The papers to be arranged are in the desk. (NOT The papers to arrange are in the desk.)

The thing is, passive infinitive is normally used after be forms (am, is, are):

       » Robots are too complex to be built.

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