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Why German says 'meines Wissens' without a preposition

German has a small set of fixed expressions that are nothing more than a noun phrase in the genitive case: meines Wissens (as far as I know), meines Erachtens (in my opinion) and mittleren Alters (middle-aged). There is no preposition, no article in front and no extra word to hold the phrase together. The genitive ending does all the work.

English speakers find this strange because English almost always needs a little linking word: as far as I know, in my opinion, of middle age. The natural reflex is to translate word for word and produce something like in meinem Wissen or mit mittlerem Alter. Neither is German. Learn these phrases as frozen blocks, endings included.

Word list
meines ErachtensB1 in my opinion meines Erachtens = in my opinion; not: meines Erachtens nach mittleren AltersB1 middle-aged ein Mann mittleren Alters = a middle-aged man; not: mit mittlerem Alter meines WissensB2 as far as I know meines Wissens = as far as I know; not: in meinem Wissen
How it works

In these expressions the genitive itself says "in this respect" or "of this kind". You can see it in the endings: meines and Wissens both carry the genitive -s, and mittleren carries the genitive -en. Because the meaning is already built into the case, adding a preposition is not just unnecessary, it is wrong.

The phrases are fixed, but the possessive can change to match the person: meines Wissens, unseres Wissens, ihres Erachtens. Everything else stays the same.

Rule of thumb: if the phrase starts with a genitive form like meines, eines or guter, do not add anything in front of it or after it.

The words
meines Wissens

Meines Wissens means "as far as I know". It is a bit formal. In everyday speech many people simply say soweit ich weiß.

Meines Wissens ist das Museum montags geschlossen.
(As far as I know, the museum is closed on Mondays.)
Unseres Wissens gibt es keine andere Lösung.
(As far as we know, there is no other solution.)
meines Erachtens

Meines Erachtens means "in my opinion". It sounds more formal than meiner Meinung nach and is common in essays, meetings and reports. It can open the sentence or sit in the middle.

Meines Erachtens ist der Plan zu teuer.
(In my opinion, the plan is too expensive.)
Die Regel ist meines Erachtens unnötig kompliziert.
(In my view, the rule is unnecessarily complicated.)
mittleren Alters

Mittleren Alters describes a person as middle-aged. It usually follows the noun directly, but it can also come after sein.

Ein Mann mittleren Alters stand vor der Tür.
(A middle-aged man stood at the door.)
Die meisten Teilnehmer waren mittleren Alters.
(Most of the participants were middle-aged.)

The same pattern shows up in other fixed phrases such as eines Tages, letzten Endes, guter Laune and schweren Herzens.

Eines Tages wirst du das verstehen.
(One day you will understand that.)
Watch out

The most common mistake, even among native speakers, is meines Erachtens nach. It mixes two different phrases. Either use the bare genitive meines Erachtens, or use the dative with a postposition: meiner Meinung nach. The nach belongs only to the second one.

Meiner Meinung nach ist der Plan zu teuer.
(In my opinion, the plan is too expensive.)

The same goes for meines Wissens nach: drop the nach. If you really want a preposition, switch to the dative: nach meinem Wissen is acceptable, though less elegant.

With mittleren Alters, you may also hear im mittleren Alter, which is correct but has a different case and a preposition. Do not blend the two into im mittleren Alters.

Summary

Some German expressions are just a genitive noun phrase, with the meaning carried by the case endings: meines Wissens, meines Erachtens, mittleren Alters. Do not add a preposition in front, do not add nach after them, and keep the genitive endings exactly as they are. Only the possessive changes to fit the speaker.