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not been compromised but he doesn’t feel its emission because he
is paralyzed – and in his condition he wouldn’t even feel the prick
of a needle because of the severance of the connection between the
nervous system and the lower half of the body – perhaps in such a
case his semen will confer impurity. This is especially relevant because
the question mentions that during the emission the patient feels some
kind of dizziness or lightheadedness and perhaps this qualifies as
having sensation.

2. A second objection is that according to what I have heard from
physicians my impression is that nowadays, even if semen is emitted
without sensation it is still capable of fertilization. This apparently
contradicts Shmuel’s statement in the gemara (Niddah 43a) that if
he does not feel the displacement or emission of the semen, it is not
capable of fertilizing.

It might be possible to say that natures have changed, as we find in
regard to many other areas where natures has changed.4 For example,
the gemara in Bechoros (44a) states that a male has two openings,
one through which he emits semen and the other through which
he passes urine. Nowadays physicians attest that there are not two
ducts. The Chazon Ish writes (Yoreh De’ah 155:4) that natures have
changed in this respect, just as we find they have changed in regard
to the einunisa d’varda. [In the sefer, Chelkas Yaakov (Vol. II, 22), it
is written that even nowadays there are two ducts, within the body
before they reach the prostate gland separated by a thin membrane
– according to this there is no need to say that natures have changed
in this regard.] The gemara states further that a person whose legs
have been severed above the knee is a terefah, yet we encounter many
such people who live normal a lifespan, forcing us to conclude that in
this respect too, natures have changed. If we assume that nowadays

4. For further discussion of this topic see earlier siman 105, regarding eye infections
in babies in relation to cirumcision, where we cite the view of my father-in-law
Rav Y.S. Elyashiv zt’l, on when physicians can be relied upon to determine that
natures have changed. See also siman 184, regarding a premature baby, born less
than six full months from conception and the Chazon Ish’s view ibid. Note 3.

28 1 Medical-Halachic Responsa of Rav Zilberstein