EXHIBIT 5.1
 
                                [STB LETTERHEAD]
 
                                                                    June 2, 1998
 
Cognizant Corporation
200 Nyala Farms
Westport, Connecticut 06880
 
Ladies and Gentlemen:
 
    We have acted as counsel to Cognizant Corporation, a Delaware corporation
(the "Company"), in connection with the Registration Statement on Form S-4 of
the Company (the "Registration Statement"), being filed on the date hereof with
the Securities and Exchange Commission under the Securities Act of 1933, as
amended, relating to the proposed issuance of shares of Common Stock, par value
$.01 per share, of the Company (the "Common Stock") in connection with the
Agreement and Plan of Merger, dated as of March 23, 1998 (the "Merger
Agreement"), among the Company, WAC Inc., a Delaware corporation and a wholly
owned subsidiary of the Company ("Merger Sub"), and Walsh International Inc., a
Delaware corporation ("Walsh"). Subject to the satisfaction or waiver of the
conditions set forth in the Merger Agreement, Merger Sub will be merged with and
into Walsh (the "Merger") and each outstanding share of common stock, par value
 .01 per share, of Walsh will be converted into shares of common stock (the
"Shares"), all as more fully described in the Registration Statement.
 
    We have examined, and have relied as to matters of fact upon, originals or
copies, certified or otherwise identified to our satisfaction, of such corporate
records, agreements, documents and other instruments and such certificates or
comparable documents of public officials and of officers and representatives of
the Company, and have made such other and further investigations, as we have
deemed relevant and necessary as a basis for the opinion hereinafter set forth.
 
    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, the conformity to original documents of all documents
submitted to us as duplicate or certified or conformed copies, and the
authenticity of the originals of such latter documents.
 
    Based upon the foregoing, and subject to the qualifications and limitations
stated herein, we are of the opinion that the Shares have been duly authorized
and, when the Shares shall have been issued in accordance with the terms of the
Merger Agreement, the Shares will be validly issued, fully paid and
nonassessable.
 
    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, the federal law of the United States and the Delaware General Corporation
Law.
 
    We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm appearing in the
Registration Statement under the caption "Legal Opinions."
 
                                          Very truly yours,
 
                                          /s/ Simpson Thacher & Bartlett
                                          SIMPSON THACHER & BARTLETT