EXHIBIT 8.1
 
                              [RMHM&K LETTERHEAD]
 
                                                                    JUNE 2, 1998
 
The Board of Directors
Walsh International Inc.
105 Terry Drive
Newtown, Pennsylvania 18940
 
Ladies and Gentlemen:
 
    This opinion is delivered to you in connection with the Proxy Statement
dated June 2, 1998 of Walsh International Inc., which forms a part of the
Registration Statement on Form S-4 of Cognizant Corporation filed with the
Securities and Exchange Commission on June 2, 1998 (the "Registration
Statement"), under the Securities Act of 1933, as amended (the "Securities
Act").
 
    In that connection, we have examined the Registration Statement and such
other documents, and we have made such other and further investigations, as we
have deemed necessary and appropriate for the purposes of this opinion. In such
examination, we have assumed the genuineness of all signatures, the legal
capacity of natural persons, the authenticity of all documents submitted to us
as originals, and the conformity to original documents of all documents
submitted to us as certified or photostatic copies.
 
    Based on the foregoing, and subject to the qualifications and limitations
stated herein, we hereby advise you that in our opinion the statements made in
the Registration Statement under the caption "United States Federal Income Tax
Consequences," accurately set forth the material United States federal income
tax consequences of the Merger to the stockholders of Walsh International, Inc.
under existing federal income tax law.
 
    We are members of the bar of the State of New York, and we do not express
any opinion herein concerning any law other than the law of the State of New
York and the federal law of the United States.
 
    We consent to the filing of this opinion as Exhibit 8.1 to the Registration
Statement and to the use of our name under the captions "United States Federal
Income Tax Consequences" and "Legal Opinions." By giving this consent, we do not
admit that we come within the category of persons whose consent is required
under Section 7 of the Securities Act.
 
                                            Very truly yours,
 
                                            /s/REBOUL, MACMURRAY, HEWITT,
                                             MAYNARD & KRISTOL