He devoted his thesis to a metaphysical concept that is difficult to define precisely.
The biblical concept of "glory" is difficult to define.
It is very difficult to define when gambling becomes a problem.
It is difficult to define poverty with a limited number of dimensions.
The liberty of a person is a concept that is difficult to define.
Richard's other character traits are even more difficult to define.
Les autres traits du caractère de Richard sont encore plus difficiles à cerner.
This makes it sometimes difficult to define the exact eye colour.
Il est par conséquent difficile à définir la couleur exacte des yeux.
It may be even more difficult to define when discussing pesticides.
Il peut être encore plus difficile à définir en relation avec les pesticides.
His manner was very difficult to define because it was changing constantly.
Sa manière était très difficile à définir car il évoluait en permanence.
Religion is a highly personal matter and difficult to define.
La religion est une question des plus intimes et difficiles à définir.
You have quality that is difficult to define, that's all.
Vous avez une qualité difficile à définir, c'est tout.
The pollution thermal has effects difficult to define with precision.
La pollution thermique a des effets difficiles à définir avec précision.
The subject, however, is sometimes more difficult to define.
Le sujet quant à lui est parfois plus difficile à définir.